This is mostly my own interpretation and opinion, but you have a wheel when it is directly in front of you. I usually follow that wheel through turns to avoid being the guy who screwed up his line and took out a bunch of others.

If your rear wheel is next to someone else's front wheel, you have the upper hand and can squeeze him out gently (gently means not chopping his wheel or forcing him into someone else). If your front wheel is next to someone else's rear wheel, it's self preservation to give in, although closing the gap and offering your elbow if he continues into you is an option.

Forcing your way into a pack is a cheap shot, but if you can find or make a hole, go for it. Aside from that, I'd say you either need to fade to the back or charge up front. The front is a nice choice because it usually doesn't take long for someone else to take the front, allowing you to sit in the second row for a bit.