Arthropod Ecology wrote a great post on Ten tips when asking for a letter of recommendation.
What I would add: Be prepared to write a draft yourself!
All advisors are busy and for them it’s a lot of help if you draft your own letter. This is harder than it sounds. Most of us instinctively undersell their own performance. But if gives you the opportunity to define which particular feature of your CV or work the letter should focus on.
And if you draft the letters for more than one of your advisors, you can actually stress different bits in different letters to make sure they don’t all sound the same and cover your general awesomeness comprehensively.
Update 4/5/13: Make sure that the letters still look unique, even if you drafted them or supplied a list of bullet points. It looks a bit weird if all your letters contain the same sentences. And don’t trust your letter writers to reformulate what you sent them. Like all of us they will be busy (and/or lazy) and most likely just copy/paste what you sent them. So it’s up to you to feed them bits that sound different from each other.