I can think of two ways:
1) Write a good answer (one that gets upvoted) to such a question .
2) Upvote an existing answer that is good (if this is genuinely the case).
Is there anything I have missed?
For questions that remain unanswered, you can try:
For sub par questions and/or questions that are essentially unanswerable, downvote and vote to close / delete. Removing the cruft is as good an option as any for reducing the backlog of unanswered questions.
The other alternative I can think of is offering a bounty for the question.
Otherwise, editing the question to make it clearer or to correct the used tags (in the case they were not correct, or missing a more important tag) could get some user to answer it. The latter would have the effect of exposing the question to the right people who can answer it: Suppose that a Drupal question is using php but not drupal; the question has not been probably noticed from those users who normally answer to Drupal questions, if not in the case they also keep an eyes on PHP questions.