Here is what I do to help prevent this. First of all I always ban evil alignments (I also find this helps the party work together better). Second I encourage the party members to write a background and reward them with xp when they do write a background (this not only helps them to have more personalty in their pcs, but also gives me great ideas for the campaign, which make a personal difference for the pcs). Third I reward players for unique ideas and playing in character with a small xp reward (about 25-50 xp, it doesn't sound like much, but it really gets my players excited and makes them think harder about whats going on in the game and gives them a mind set of "What would my character do in this situation?")
This has worked very well for me, I hope it helps!
This has worked very well for me, I hope it helps!