This is going to be a fairly personal perspective (and thus may not be directly applicable to situations that aren't very similar), but my feeling is that VCs really trust projections when you've shown them the previous projections you've made over a number of months/years against the actuals. For example, in our recent pitch process, we showed 2009's estimated revenues/expenses/etc. alongside our actuals and this certainly helped to show that we're serious when we say we A) always estimate conservatively and B) always beat projections.
When you can show things like visits, conversion rates, retention, recitivism, etc. in a systematic way over time, then make projections that estimate slowing down your growth (i.e. "We're not even going to improve next year as much as we did this year") that makes for, IMO, a compelling, believable story.