This is a subject that comes up a lot in lessons. I have an exercise that I devised to help me solve the issue.
The Adobe Reader files I link to here show you how to do it. Basically, it is playing a B major scale in 2 against 3 and 3 against 2, then in 3 against 4 and 4 against 3. Initially, they should be practiced slowly with accents on each beat. Then gradually faster and gradually removing the accents.
If you have real trouble feeling the placement of the notes, you should do one beat at a time. In other word, from the first note of one beat, up to and including the first note of the next beat. This should be done with the right hand, left hand, then together, over and over again until the together part sounds like each hand is independent of the other. You really have to feel each hand’s separate rhythm very strongly when each plays alone.
Here are the files: