How long to wait before sending a follow-up email asking for a letter of recommendation? I am currently applying to graduate school and trying to gather some solid letters of recommendation.

During my undergraduate studies, I worked in a research lab and while I didn't get that close with the professor, I decided to take a shot and ask her for a letter of recommendation. I sent an email two weeks ago and she has not responded. How long should I wait before emailing again? How do I ask again without being pushy? My fear is that she barely remembers me and this is her way of saying no. Should I just ask someone else?
Professors receive many requests for recommendations.  Speaking for myself, I appreciate reminders because it lessens the organizational burden of keeping track of everything.

As a rough guideline I would wait 2 weeks between requests (so, go ahead and try again now).  Just be polite and re-send the request with a note to the effect that you know she is busy, so you are re-sending your request.

I would also add that if you aren't close with this professor it may be hard for her to remember enough about you to write a strong letter in support of your application.  To remedy this, it would be helpful if you also reminded the professor about what you did for her/in her lab when you contact her again.