By accepting an answer you're obviously already saying that it was useful. Should an additional up-vote be reserved for answers that are particularly useful, or do people always tend to use the two in unison (or the contrary)?
I upvote any answers that are helpful: perhaps they're not the exact solution, but they're alternatives, or on the right path.
Sometimes there are duplicates. If duplicate answers should be 'accepted', I prefer them to be upvoted to match each other, or allow one answerer to retract his answer in favor of the other guy.
But overall, if you're accepting something you're not upvoting, why are you accepting it?
The answer that is accepted by the user solves the user problem. Meaning they should have either tested it or it solved their problem. If the question is going to accepted by the user it should be up voted by the user.
Upvote any answers that helps add to the knowledge of the question, they may not have the perfect answer per se but help on the path of the perfect answer.