Right people have given some quite complicated answers to this, this is a method that'll work for all polyrhythms (2 against 3, 3 against 4 etc.). It's very simple.
First take the two numbers, 3 against 4, times them together to make 12. Then right out 1-12 on a piece of paper. To make them the same size I used 0, 1 + 2 again at the end.
CODE
1 2 3 4 5 6 7 8 9 0 1 2
Every 3rd beat put an x. staring on first:
CODE
1 2 3 4 5 6 7 8 9 0 1 2
x x x x
And do the same with every 4th beat:
CODE
1 2 3 4 5 6 7 8 9 0 1 2
x x x x
o o o
Now all you do it count aloud: 1(Both) 2 3 4(left) 5(right) 6 7(left) 8 9(right) 10(left) 11 12
Do this very slowly at first and build up speed, eventually you will be able merely to 'feel' the rythm and you won't have to count it in your head.
If you'd have done it for 3 against 2 you'd have
CODE
1 2 3 4 5 6
x x x
0 0
1(both) 2 3(left) 4(right) 5(left) 6.
This does work for them all, but some of them are quite long winded, eg 7 against 4
CODE
1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8
x x x x x x x
o o o o
I must confess I don't really know what you'd do in that situation, perhaps in time that would seem natural as well, I've never been bored enough to try and count to 28 for fun.
I find it's easiest to get the rhythm right, either by just doing octaves on piano, or simply by tapping your hands. And then you'll find actually playing the notes in rhythm isn't that hard at all.