I read all of this with great interest - thanks for starting a super helpful thread, and thanks to all the people who posted great suggestions! I've certainly had similar problems, especially with guys who like to fuck deep. I do the best I can.
I will add that, because I have a rather difficult time finding tops at all in my area, it is hard to plan ahead, and pointless to take a long time doing this (because the tops, when you can find them, tend not to be patient). But it's vastly important to take care of yourself, and the whole point of the exercise is to enjoy, right? Added stress reduces enjoyment, and is not good for us! Therefore, my suggestion (which I try to live by) is: Do your best, but don't wig out with stress over the fact that sometimes it won't be perfect. Shit happens. Literally. It's just messy, not the end of the world. So use a towel in case it does, and apologize if it does - but casually, and not as though anything is wrong. Because nothing is wrong. Your body is working just the way it is supposed to. If the top doesn't like that... too bad! He chose to play, with full knowledge of the possible consequences. I find when I can live by this plan, it makes me much happier... and actually I don't think I've ever had a top get really bent about it.
Keep Calm and Keep Fucking