Here's what you do:
First off, when you ask, ask if they will feel comfortable writing a good LOR. If they show signs of hesitation, aren't enthusiastic, or flat out say no then move on to someone else b/c you're not going to get a great letter from them.
Secondly prepare a packet for them. It should include the following:
Cover letter - Thank them for writing a LOR and tell them why you think their insight will be important. Also ask them to write about certain things that you want them to write about. Tell them who you're other letter writers are and what they're writing about. This way each letter talks about a different aspect of you and you don't have x number of homogeneous, interchangeable letters.
A copy of your CV - Straight forward. To give them a sense of your activities that they might not be aware of.
A copy of your transcript
A copy of your PS - It can be a rough draft, in fact thats a good thing. Every week or so, give them and updated copy. That way you get feedback on your PS revisions and you are reminding your prof that he has to write your letter without being blatantly annoying.