Is Evil Gaz's advice still around somewhere? That was always worth a read.
My advice is that if you do write pre-gens, give them a reason in their backgrounds to spark off each other. I'm not advocating PVP, but you can give them a steer on how they get on. A couple of hooks mean that the players tend to engage in more. Of course, some of the more modern games do this by design (e.g. bonds, relationships etc).