I started out joking about it, just to gauge level of interest and/or thoughts on it from the other person. Once you know where you're starting it gets a lot easier, since you know how far you have to go. I would just say take it slow, ask politely and without force, plant the seed in the other person's head. Then, do nice things for them (both in and out of bed), not with the expressed intension of getting anal, but knowing it could be a possible reward. If it doesn't happen, it doesn't happen, just make sure your partner is in for the decision making process.