I say "yes" way too often and regret it almost immediately.  I found this great post on Nick Armstrong's blog about "Learning to Say No" offering this great way to narrow down the things to say "yes" to.  Using this 5 lettered word reminder, may help.

Nick says  "I think this week, I wanted to give myself an easy toolkit to remember how to say no. So I’m going to start asking myself the WHORE question.

Who is it for – Some people get more leeway than others.
How much time – 5 minutes is still an interruption.
Overwhelmed – If I take this on, will I feel overwhelmed tomorrow?
Return on investment – Do I need the money? Will it open doors? Does it serve some intrinsic need?
Enjoyment – Why agree to do something you don’t enjoy? (“we all have to do things we don’t like” is not a valid excuse).

If more than half of the resulting answer is unfavorable, I’m not going to take on the task."