Well, normally it's a new player. old hands don't forget. I'll remind the 1st time, and maybe once more, but that's it. It's a valuable lesson learnt, after that.
Your opponent forgets to press his clock

I believe, that as long it isnt against the rules, use all your tricks! I always do what is best for me in any way, to let your opponment go down psychologically is always good for you!
If my opponment forget to press his clock, i find my move, and when hes pressing it, i do my move immediatly. This may annoy him a bit, and thats great for me! Plus i got some extra time = win, win.

in the area there I play chess it is within good sportmanship not to notify your opponent that he forgot to press the clock.
if he forgets I will sit on my hands until my opponent finally remembers to press the clock. I will never make a move.
actually until he has pressed the clock it is still his turn and he can still offer a draw.

Because after all, chess is war!
No, its not. It is a game where you push little wooden pieces around a table.
I would say "Your clock..." as I considered my move, that seems the least disrupting to my own thought process. After 2 or 3 times it is not my problem.
BobbyDK's suggestion is a good one, that its still his turn until he presses the clock. Squeezes a few more seconds out of him, too. On the other hand you might spend five minutes just sitting there! Depending on your attitude and the prize money, might be a good or bad thing...

Well, normally it's a new player. old hands don't forget.
In the tournaments I played even experienced older players forgot to press the clock.
But a bigger problem is those who hit on your clock instead of their own if they sit and play on the same table next to you. you may not notice it and it may appear as if you haven't pressed the clock.
it happend to me and I finally found out and called TD.

When I played chess for my high school I would usually just say "clock" every time they forgot. It was supposed to be a "friendly learning environment" and I was sometimes playing against very young players so I didn't really mind too much.
In a tournament for a prize I don't think I'd remind them more than once.

Well, normally it's a new player. old hands don't forget.
In the tournaments I played even experienced older players forgot to press the clock.
But a bigger problem is those who hit on your clock instead of their own if they sit and play on the same table next to you. you may not notice it and it may appear as if you haven't pressed the clock.
it happend to me and I finally found out and called TD.
This particular problem is often due to too crowded playing conditions, a lack of space. I have found this is a much more common problem in Europe than it was when I was playing in the USA.

As for reminding the opponent about the clock , I give only one reminder , if I don't dislike my opponent , and none if I do dislike them.
The only time i will say anything is if it is clear that this is their first OTB tourn. Any other time it is not my responsibility to watch your clock for you. Some people may view that as rude but the clock is part of the game.

Well, normally it's a new player. old hands don't forget.
In the tournaments I played even experienced older players forgot to press the clock.
But a bigger problem is those who hit on your clock instead of their own if they sit and play on the same table next to you. you may not notice it and it may appear as if you haven't pressed the clock.
it happend to me and I finally found out and called TD.
This particular problem is often due to too crowded playing conditions, a lack of space. I have found this is a much more common problem in Europe than it was when I was playing in the USA.
It happened to Kasparov in a World-Championchip-Match against Karpov when he forgot to press the the clock for more than 80 minutes.

I've had it happen a few times. Whatever you want to do, is my opinion. It's not hurting anyone to allow your opponent an error during a chess game.

I think even the arbiter is not allowed to notify the player if he has forgotten to press the clock. but I don't know the rules 100%.

It's a gentlemen's game, so do you also point out a hidden danger of your move? What if it looks like they're about to fall for something?
If I notice they haven't started my clock, like Ziryab I'll just let it tick. I don't try to take any longer than usual to decide on a move (I guess that's my interpretation of being a gentleman about it heh), and after I move I don't touch the clock as a visual indication that they forgot.
In a blitz game I'll remind them once or twice -- then not at all. Again I'll just move without touching the clock and they usually figure it out.
Well, normally it's a new player. old hands don't forget. I'll remind the 1st time, and maybe once more, but that's it. It's a valuable lesson learnt, after that.
I once lost to an 85 year old part time magician who was an incredible chess player. He was solid as a rock. Alas, his rating was hampered because he mostly forgot to press his clock, unless it was a particularly energetic move.
But, the most heartbreaking was when he put his piece en prise, just because his mind was thinking a square, but his arm coudn't quite gauge the right distance.
Used to happen about once every 2 games.
Bless.
In over the board chess I usually point it out to him because after all, chess is a gentlemen's game. When my position is bad the compulsion to tell him is not so great and I begin to rationalize, "it's not my responsibility to manage his clock...besides having to point it out to him is distracting to me." Maybe he'll lose on time or blunder in time pressure! Because after all, chess is war! Your thoughts...Is not informing your opponent that he forgot to press his clock unethical?