In addition to what louannePiccolo said .... as a general rule, the longer the phrase is, the more polite it is. So - the following go from most direct to most tentative :
Could you stand up?
Do you think you could stand up?
I was wondering if you could possibly stand up?
However, it's not just the forms themselves but also the structure of the whole conversation. For example, if I want to ask you to do something, I'll probably start what's known as a pre-sequence. - Jean, do you think I could possibly ask you a favour?
- Yes, sure. What can I do for you?
That warns you that a request is coming and gives you a bit of time to think of an excuse if you don't really want to help. I give you even more time by "spinning out" the request and prefacing it with an explanation : -Well, the thing is, you see, I'm going on holiday next week and I was wondering if you could possibly look after the cats for me?
- Oh well, I'm really sorry but, erm, I'm afraid I'm horribly allergic to cats.
Notice that things like the hesitations, and of course the intonation, all contribute to the tentativeness and therefore the politeness. With a different intonation you can make any polite form much more aggressive. The use and position of the word please
contributes to this too. Look at the following situation :
A mother speaking to a child :
Alex, can you go to bed now, please?
(10 minutes later nothing has happened)
Alex, could you PLEASE go to bed now!
(10 minutes later and Alex is still at the computer)
could you go to bed!!
You can find all this explained in a lot more detail here