I have asked a question and received a good response. But I want more information about it. How am I supposed to go about asking it? Just a create a new question?
You can reply to comments, that's a good start.
You can also edit your original question asking for clarification.
You could always add a comment to the answer asking for more information.
I think modifying the original question after there are already many answer can lead to very confusing threads; it can make the highest voted answer either inaccurate or so it answers only a piece of the new question. I think the best way is to either comment on the answer that you want more information on or if what you want more information on is detailed enough, asking a new question that references the first.
You can use a bounty to attract more users to your question or to request better quality answers.
Digging a six year old question from the grave! :] I don't know when bounties were introduced to the Stack Exchange network, but I'm pretty sure that it was after this question and all of its answers were made. So this answer is a little bit of an update.
Also, there are tags called "needs-more-answers" and "needs-answer"