There are three types of favours one might ask of a friend: Unacceptable requests, acceptable requests, and the grey area between the two of them. Acceptable requests are fine to just ask, unacceptable requests shouldn't normally be asked. Here I am interested in grey area-type requests: stuff that is not unacceptable, but that might place so heavy a burden on the askee that you wouldn't be cross with her if she refused.
Let me propose a norm for how you should go about if you've decided you still want to ask the favour. Your aim should be to make it as easy as possible for the friend to say no. This means you want to do the opposite of what's done by a power salesman, who tries to get people to say yes. A power salesman will try to get as close to you as he can. Ideally, he wants you in person; the telephone is the next best thing. Conversely, you should try to keep a distance from the askee when you ask and when she gives her answer. This means you put the request in writing: e-mail, letter, fax - I don't care.
Also, including something along the lines of "of course you can say no" helps.