A tip that seems to work for many people when bending while drawing is to "pronounce" (you don't use your voice, just imagine you would) "eeeee" and "ooooo" (English pronunciation) in an alternating fashion, the former corresponding to the normal note and the latter to the bent note.
I also found that you need to be sensitive (but that might just be me) to how high you are on the harp. The third and fourth holes are the easiest for me, the second requires me to slightly move my jaw forward, and for the sixth hole it works the other way around: if you visualize your tongue as a curve, then the "peak" of that curve should be closer to the harmonica for higher notes, and closer to the back of your throat for lower notes.
You should also check out a few instructional videos by the gods of harmonica on YouTube: Jason Ricci, Adam Gussow and Paul Lassey (in French at the time, but he's starting to make a few in English) are really helpful in my opinion.