I mostly vote for the person who best matches my beliefs and appears to have high integrity, but I adjust my approach in certain circumstances.
In spite of the folks who assume I'm a Democrat because of my criticism of Bush-lite, I actually have little respect for either party.  Both parties are sell-outs to special interests.  In that light, if a given race is lopsided anyway, I will often vote for a third-party candidate.  I figure my vote speaks louder as an expression of dissatisfaction then it would if I threw it away on a Republicrat candidate.  Similarly, if I don't have a strong preference for either of the two Demolican candidates, I often vote against the incumbent.  Same rationale, show dissatisfaction with the status quo.
This has shifted somewhat over the past few elections.  I keep finding that Republican actions outrage me more frequently than Democratic actions.  In a close race with no clear "better" candidate, I tend to give the Democrat the benefit of the doubt.