Do: Gauge her mood carefully. Pick a day when se is feeling good.
Don't: Ask on a day that she has had bad day at work, feels sick, had a bad talk with her mother, feels fat, hair didnt turn out right, feels bloated, is too cold/hot, etc.
Do: Consider giving her a nice present.
Don't: Attach the present to your penis
Do: Take her to dinner.
Don't: Take her to Red Lobster
Do: When actually asking for oral sex, make every other word "please".
Don't: Phrase your question in a disgusting way. (ie. Can I wash out your mouth with my man soup?)
Do: Beg if neccessary. (It will probably be necessary)
Don't: Wear that shirt that she hates because you were wearing it that time you were "acting like an asshole, in front of those stupid friends of yours".
Dont: Think you can get away with making out for a while, then pushing down on the top of her head.
ADVICE FOR WOMEN
Do: Ask politely (asking impolitely is fine too)
Do: Point your finger in a downward direction.