Hey I use an Ibanez Prestige but I find that after using the tremolo much the guitar just goes out of tune. I presume this happens to a lot of people so I'm wondering how Steve Vai and Joe Satriani manage to use their tremolos so much and not go out of tune?

Well a guitar with a locking nut shouldn't go out of tune.

After you put new strings on your guitar make sure you tune them and then stretch them and re-tune, etc. etc. - until stretching them doesn't make them go out of tune.

If you're still having tuning issues I'd recommend taking your guitar to a tech to have it checked out.

your guitar is not set up properly, this is why it goes out of tune.
I had this problem for over two years, so I know how it is.

It would be the best advice that I can give you is to go to a person that knows about guitars, tell him that and askk him to fix it and explain to you how to do it yourself (you cannot go to someone every time you decide to buy strings from a different brand) and most importantly - show it to you.

I learned everything the hard way and I am happy now but for two years I hated the tremolo and could not even touch it. In the end I fixed it with.... vaseline :D:D:D

Having this knowledge is really important so try reading about it here:
http://www.ibanezrules.com/tech/setup/index.htm
http://www.ibanezrules.com/tech/setup/tuning_stability.htm (here some are about adjusting the tremolo)
http://www.jemsite.com/tech/1setup.htm a really good with pictures!!!

There are many many various places where you can find info on how to do it.
Above, the third one has really good pictures on how it should look like and how to do it, only look at Step one.

If it is still unclear, ask again and I can try to exlpain it from my point of view, but I would probably repeat what is written :)

edit: I disagree with the post above. The problem here is that his tremolo is not returning to the zero point (neutrality) after being used. That has nothing to do with a locking nut. The tremolo itself is not set up properly.

2nd edit:
http://www.ibanezrules.com/tech/setup/sharpening.htm
this is also the cause! I did both dead tuning and the shapstick of the last part from this link and it retunrs perfectly :)

I would quote the last part from that link

I didn't say it had anything to do with a locking nut. I said a guitar with a locking nut should not go out of tune - if set up properly.

there are many factors that will cause a tremolo to not stay in tune..these here are common
1)loose locking nut mounting bolts
2)Bridge pivot points need graphite, lubrication or sharpening
3)Bridge hasnt been broken in
4) when restringing..stretch the strings out until it stays in tune without the nut clamped
5) dont be afraid to tighten nut bolts securely...if an old guitar, have extra locking nut bolts so you dont strip out threads...change them regularly