I'm still pretty new to this sport and threads like this have been really helpful to me. The notes above are great. The one about making a spot look unavailable is my usual approach. REx's notes are right on as well.

One thing that I've noticed is people don't want confrontation. It is really surprising how many folks will immediately abandon a great spot if someone starts to come in. It might take less contact than you realize to tell him that the spot is yours.

For example - I had a situation last weekend where I unexpectedly took a spot from a guy. I was ~20 riders back with half a lap to go when I moved up along a single file field and pulled in tight to parallel the third rider. I anticipated being hung out to dry but the guy immediately moved off the line and let me into the queue...15 seconds from the sprint this guy gave up the most coveted spot in the entire race at the best possible moment. I wish I had noticed who it was as I felt like I owed him money afterwards.