DO NOT IGNORE THIS POST..............

ok, cross rhythms, playing 3 notes against 4 and all that stuff, it keeps coming up in all these pieces I play, even ones I wouldn`t expect. All I want is one, just ONE decent post, explaining in heavy detail how you go about doing it. PLEASE, this is torturing me, just give a step by step guide or something that explains exactly how you went about doing it the first time you did it, and then I will shut up. PLEASE, this is like, the one thing, THE ONE THING that would make my life easier than anything else. or give a weblink to some piano teaching site, ANYTHING!!!

Ok, I know how to apply this to cross rhythms with 2 against 3. I don`t know if this is gonna help you at all, but I`ll explain anyways.

First of all, forget about any piece you are playing. Forget about the notes and sit down at a table

The phrase you have to say to yourself is: "nice cup of tea". Now what you are going to do is play this by drumming on the table..

syllable 1. (nice) you slam down both hands
syllable 2. (cup) slam down just your right hand (R)
syllable 3. (of) L hand
syllable 4. (tea) R hand

Keep doing that to the rhythm of how you would say "nice cup of tea" until you start to get good. You can reverse the pattern between the L and R hand at anytime, too.

I do not know a phrase to learn cross rhythms against 3 and 4.. maybe you could somehow double the hand which is playing 2 beats

Hope this has helped

For 3 against 4 you can do the same thing as suggested in the previous post, except say "Pass the golden button".
Pass - L and R hands together
The - RH
Gol -LH
-den - RH
But - LH
-ton - RH
Just keep in mind that the RH notes don`t fit evenly between the LH notes. It takes some practice, but you can get it. Goo luck!

The above 2 posts are nice little tricks to learning how to do it, but it doesn`t really help when you`re asked to play something really obscure, like 3 on 5 (theres rachmaninoff prelude that has 3 on 5 AND 4 on 5) , now one thing you must remember, is sometimes it is actually impossible to play it perfectly in time (look at the end to chopins nocturne op.27 no. 2) and it isn`t actually meant to be played perfectly, however for the times when you want to keep in time, this is what I do:
-learn right hand using metronome (rhythm 1)
-learn left hand using metronome (rhythm 2)
Now get to the standard where you could play it with either hand while having a detailed conversation with someone, i.e. spend a LONG time doing this. Next try fitting them together with the metronome, it won`t just magically fall inot place but it will be easier, this takes time to get right so be very persistent and you WILL eventually get through it, believe me the first time I did it I thought I would never get it, but eventually it all came together. This is NOT an easy thing for most people to do, if you are a beginner, DO NOT TRY IT YET, it`s not something you need yet.