I wrote a question today that, thus far, has received five answers (plus one from me) and zero upvotes. That's fine, it happens. But, this irks me a little, someone actually bookmarked it and (unless someone downvoted me) didn't upvote the question. I realize the criteria for upvoting is "clear and useful" not "interesting enough to bookmark", but I don't think I've ever bookmarked a question without upvoting it. You obviously like the question enough that you'd like to be able to return to it later, so I think in most cases, an upvote is in order. I wonder if SO could provide one of those little pop-up reminders suggesting you upvote a question if you bookmark it.
I am thinking something along the lines of:
If you think this question is clear and useful, please consider upvoting it.
So, just a little encouragement to act nicely if it's deserved.