I find it hard to memorize opening lines in which many moves are the same and look like other lines, and I find it much easier when I give each one a funny name (the Pirate ship Defense, the pineapple attack, etc..) to give them individuality. What memory techniques do you use?

here is a good website dedicated to memory techniques.

http://mnemotechnics.org/

Here is a site with tons of links to memory techniques. They also have some interesting new methods.

http://memorymethods.com/

I think you should try and attach a kind of "character" to each move/sequence of moves. The most obvious thing would be how aggressive/audacious the move is... like the "audacious" f4. A positional move, the main line, the "old" main line. Or a move Steinitz played, or Morphy or Fischer or Petrosian. All of these things help memory I think.

And better than that is if you can say something is like another structure, like a french defence or alekhine's defence or philidor or velimirovic attack. That way you can take properties from your known opening and attach them to this new one.

CPT is a really good software for this, you load the openings as PGN and then it tests you for those lines. It has levels like Beginner,Club Player,GM so you don't have to worry if you don't know anything about the opening or if you know it really well.

Memorizing opening lines is a complete waste of time. 

Play over master games in the lines you are interested in, no more than 15-20 minutes per game BUT play them out, all the way, whether wins, losses, or draws.  You will begin to recognize the patterns and plans which work in each pawn structure - and those that don't.

In this way you learn to play the position, and you actually know something as opposed to rote memorization you will swiftly forget and won't help you once you reach the end even if you can remember them all.

GM Nigel short said something like, I cann't remember anything that I dont understand. if you are memorizing moves then your doing it wrong. Learn WHY. its a slow and sometimes mind numbing process but it pays off in the long run.

if u get the idea behind the opening, u should remember the line better

ok Training tip: After picking the opening you want to work on select ALL the games that start with that in a large database. It should get you several thousand games. Start with all the games that players win or lose in less than  20 moves. Collect those into a seperate database. select games where the winning side is at least 2000 or higher. Review at least 50 games preferrably 100 game of these games in a fast forward mode, about 1-2 seconds a move. Look for the common tactical mistakes (which are also strategical ones!) If your very motivated start collecting a database of these typical mistakes  [I name the database just that, Typical mistakes-scotch] You also start to learn about move orders, piece development and basic plans. 

You can also find out why some lines arent so bad as you might think too such as the need for h3 in the ruy lopez or why it is needed.

I like TonyH's method, but with certain refinements.  First, filter the games for the opening line AND rating.  You want games by FIDE masters on both sides, so figure 2300 for both sides.  If you still get thousands of games, filter again, depending on the variation, by date for the last ten years so you get no more than 1000, raise the rating minimum if required to get down there.

It is possible to learn from games of lower-rated players, and many non-masters have found innovations, but it is rare and they will lead you astray more than they help in training.

Also, I recommend spending more time on each game, but not deeply analyzing.  15-20 minutes is plenty of time, less for shorter games under 40 moves is okay.  But you need to look at the position after every move and see what has changed, at least half a minute if you don't know the position already, with an eye to how the central pawn structure is similar and how different players approach the game based on it. 

In this manner, you see not only the opening moves, but the typical pawn structures and typical middlegames that arise from it, and how to play them.  You will also be surprised that even the endings which arise from a given pawn structure will fit into a narrow range.  Now you are learning how to play positions, understanding them in the sense GM Short mentioned, and you will be much better prepared than just trying to memorize lines without understanding the positions.

Estragon, The review of games by more advanced players is a good way to refine your openings. The problem I found with that is that the typical traps and tactical mistakes are bypassed in the vast majority of master games. You never see the more advancd version of "oops" moves unless someone puts them in the notes or you happen to try it out against a computer.The reason we see longer term strategical plans nowadays is that the flash attacks of the romatic era have been dealt with at the master level. We have to know what traps and tricks we are avoiding so we can understand why we are playing other moves. It is difficult to see that move X on move 5 is bad until you notice a few games that players just get crushed.

Following this idea is that 1800-2000 players will probably not play overtly bad moves and not miss a simple attack (oh i didnt see you could take my queen in one move) but will make relatively deeper tactical/strategical mistakes that get punished by the stronger player with more tactical awareness, calculation skills and/or knowledge in the opening. I did this because I found that the vast majority of novice games someone deviates early on (by move 8-10 usually) and the move is superficially "ok" but found a lot of games in my database were someone played it and was punished in 15 moves. Building up this base also has the sneaky feature of teaching tactics, target consciousness and reinforcing principles.

The First stage to me in learning an opening is finding these  beginner errors. How are simple mistakes punished, often these are pawn pushes, unnecessary trades, etc.... moves that masters wont make but players at the U1500 level make on a regular basis. Create a database of these tactical mistakes now and you will also have a nice tactical file as well. One of my students faces 5....Nxd4? in the scotch on a regular basis. to give you an idea here is an opening trap that a 1900 player (USCF) fell into he ended up drawing the game because he didnt give up and took some chances 

Another trap against a 1700 player that I played