I find that the answer is often rather zen-like: You need to remove some of the bassy sound in order to get a more bassy sound.
Phase problems are more pronounced with low frequencies so if you have e.g. a bass and a kick drum that both have low frequencies, chances are you loose a lot of oomph when you combine the two in your mix. That's why EQ'ing or compressing one of the sources as Powertieke suggests will help. You will have to experiment to see whether emphasizing the bass or the kick, or whatever instruments you have works best.
Don't forget to listen to the other instruments in your mix too. Some of them might have a lot of low frequency energy too that you didn't anticipate or expect. If they don't play a crucial rule in the bass department, get rid of it too. Also, if you're recording real instruments, make sure you experiment with different micing setups before you reach for the compressor or EQ.
Another option is to arrange your instruments to accommodate each other better. For example, if you have the kick going on one and three, and a pumping bass on all four quarter notes, maybe changing the bass to slightly emphasize 2 and 4 might work. You could argue a compressor kind of does the same, but you might get a different "feeling" and richer expression when you play the instrument more dynamically.