All the time, I automatically would just lose it right after looking at (even) a basic and common opening (I just read the Zukertort Silician Invit., most likely going to forget it). No matter how many times I tried to memorize (and look at) the Sicilian and French variations, there will be the 85% of them slipping away the day after.

No, don't tell me to write them down (I did for the Alapin and Dragon Acc, but not working much). My weakest area is on the openings (because it's sometimes suggested that one should work on the middlegame and endgame first?), and often will struggle with them, and if I do, it would be God's hand that I manage to win the whole game later on.

I think its because you're not playing those openings, you should play online chess ( the one that takes weeks to complete ) and use your openings. Start about 10 of them and slowly memorize them. That would be more beneficial to you because you play your openings at the same time in online chess, you can look at an opening book and thats not considered cheating.

I play correspondence and a little casual OTB and suffer from the same problem.  Two thoughts (neither of which is original):Blitz is good for testing, and each game if you learn 1 move further in that line; hey, you're making progress for a short investment in time. Of course some don't want to sacrifice their rating in what is really an ongoing experiment.  You can even play blitz against a computer for that matter. HIARCS lets you start from any of a few hundred common openings.

Second, maybe all those titled players bemoaning the loss of time spent on opening theory are right. Maybe 5 to 8 moves deep is all we need.  Maybe we need to learn how to outplay our opponents instead.

EDIT: Sorry, woke up in a "preachy" mood this morning. 

I agree with baddogno, my repertoire is small and I don't go into deep variations. I suppose it's easier to memorize after having enough experience playing those openings. But I try to at least take a look on every possible scenario: I don't want to figure how to answer the Polish over the board.

Meditation make you better at chess. Here is a website that has some good meditations. www.JoyofSatan.org

Why do I forget the openings so easily?

Maybe the simplest answer is the correct one.

You forget openings so easily because you have no interest in them.

Most people forget things they do not care about/have little interest in.

The opening is the easiset part of the game.  Quit trying to remember moves, that dont explain why moves are made.  If you want to get a deeper understanding of the openings you want to play, study, and learn the pawn structure associated with those openings.  

In a 2007 GM John Nunn book, in connection with opening study, it is stated that, if a "book contains illustrative games, it is worth playing these over first", and the reader was also advised, "To begin with, only study the main lines - that will cope with 90% of your games, and you can easily fill in the unusual lines later."

In one of his books about an opening, GM Nigel Davies wrote (2005), "The way I suggest you study this book is to play through the main games once, relatively quickly, and then start playing the variation in actual games. Playing an opening in real games is of vital importance - without this kind of live practice it is impossible to get a 'feel' for the kind of game it leads to. There is time enough later for involvement with the details, after playing your games it is good to look up the line."

I still vividly remember the time I faced 1 e4 e5 2 d4 exd4 3 c3 over the board and I discovered that I had completely forgotten what Reinfeld had told me to do about it. After getting clobbered, I looked up the book recommendations and I think that I am considerably more likely to remember at least some of that stuff if the situation ever arises again. For many players, I suspect that their opening knowledge expands gradually in that sort of fashion. There are many sample games in Openings for Amateurs by Pete Tamburro (2014) http://kenilworthian.blogspot.com/2014/05/review-of-pete-tamburros-openings-for.html

Because you didn't study as thoroughly as you should and you didn't pick up the reasons behind the nuances.  Here's a game of mine as black:

The variation I drew "inspiration" from was this one:

I see, thank you very much, maybe I can consider that.

That's another problem, to actually "understand" the opening, I can forget that part as well for some stupid reason. But I'll try.

In a 2003 book, FM Steve Giddins wrote, "... one simply cannot play the [Najdorf Sicilian] safely without studying the complications and remembering a lot of concrete variations. If you are averse to doing this, or you have a poor memory, you are better off avoiding such lines. ... [There are main-line openings] that require less concrete knowledge and are based more on understanding." Consequently, I think that there can be some value to BROWSING through a book like Back to Basics: Openings by FM Carsten Hansen (2010) http://www.thechessmind.net/blog/2010/1/30/a-brief-review-of-carsten-hansens-back-to-basics-openings.html in order to try to identify openigs that require less memory to prepare.

In a 2010 book, GM Andrew Soltis wrote, "... good books can [give a clear verbal description of what an opening is all about] ... If there isn't any text - if it's all just moves - then this isn't the book for you." John Emms made a particularly extensive effort to use verbal description as part of his explanation of basic opening principles, in his 2006 book, Discovering Chess Openings https://web.archive.org/web/20140627114655/http://www.chesscafe.com/text/hansen91.pdf .

It is a waste of time anyway because your opponent will play a move not even mentioned in the opening books.

you could try http://memchess.com , it gets you to memorize the common lines by going over them again and again. its a bit crappy though

If you are "forgetting" openings, you aren't learning them properly in the first place.

If you actually study an opening and proceed to understand the purpose of each move, then even if you forgot one move, you should easily be able to figure it out because you actually "understand" the opening and the whole purpose behind it along with the upcoming middlegame motifs.

If all you are doing is "memorizing" lines, you ain't learning jack!

If after 1.e4 e5 2.Nf3 Nc6 3.Bb5 a6 4.Ba4 Nf6 5.O-O Be7 6.Re1 b5 7.Bb3 d6 8.c3 O-O 9.h3, you are unable to answer ALL of the following questions, you don't understand the Ruy Lopez at all, all you did was memorize a line of moves, and the moment your opponent deviates, you are screwed!

1) Why does White not win a pawn after 4.Bxc6 and 5.Nxe5?

2) Why move 6 for Black's ...b5 push?  Can he wait longer?  Why not just immediately chase the Bishop with 4...b5 and 5...Na5?

3) Can't Black just take on e4 on move 5 and be up a pawn with no consequences?

4) You are Black and White says "Now why would I make such a stupid move like 9.h3?  Just get on with the program and play 9.d4."  Do you know why 9.d4 is considered inferior to 9.h3?  What move should Black make after 9.d4?  WHY should Black make this move?

If you can't score 100% on those 4 BASIC questions, you don't understand the Ruy Lopez at all.  I don't care if you can regurgitate the moves.  You don't know anything.  And therefore, you are memorizing, and if you "forget" as the OP mentions, forget it, tip your king!  If you actually "understand" the basis behind the whole system, even if you forgot whether to play 8.c3 or 8.h3 or 8.Na3 or 8.g3, basic comprehension of the position should lead you to the correct move, 8.c3. 

IMHO the best way to learn openings is to concentrate on the openings in your chosen repertoire. Play games with your repertore blitz games, online, games, whatever. When the game is over analyze the game using an opening book or computer program or opening explorer or all three. Decide what you did right with that opening and what you did wrong and why. Remember the changes you want to make when you play that opening the next time. To help yourself remember those changes type them into notepad and save on your desktop for future consultation.  

1) Why does White not win a pawn after 4.Bxc6 and 5.Nxe5?

Because it doesn't win a pawn black has the response Qd4 or Qg5 regaining the pawn.

2) Why move 6 for Black's ...b5 push? 

Black plays 6...b5 because white is threating to win the e5 pawn now. The move Qd4 or Qg5 no longer work.

Can he wait longer? 

No black can not wait longer. He must deal with the threat in some way.

Why not just immediately chase the Bishop with 4...b5 and 5...Na5?

Because the black e5 pawn is not yet defended sufficently.

3) Can't Black just take on e4 on move 5 and be up a pawn with no consequences?

No white can regain his lost pawn. White has the strong reply 6.d4 which regains the pawn back.

4) You are Black and White says "Now why would I make such a stupid move like 9.h3? 

It defends the g4 square. Which prevents a black piece from going there.

Just get on with the program and play 9.d4."  Do you know why 9.d4 is considered inferior to 9.h3? 

Because whites d4 pawn can come under fire.

What move should Black make after 9.d4? 

9...Bg4

WHY should Black make this move?

It pins the knight and indirectly pressuring the d4 square.

If white plays foolishly black can take on f3 and white would have to retake with there pawn and be left with messed up king side pawns or down the d4 pawn.

4/4   100%  I am the 

Wooooo Hooooooo Ruy Lopez Master Woot Woot

What about you guys/gals.