Sometimes I want to move with it (sometimes involuntarily), but I'm just not sure what the girl will think. I also don't know what to do about ejaculation. I mean, it's not like I want to have to take off my shirt or lift it up. I also don't want to get semen everywhere and then have to go through the clean-up process.
I'll tackle your concerns one by one. First, it's perfectly normal to want to move your body when someone is masturbating you. This should be a time when your partner is giving you pleasure – so there's no need to destroy the mood by worrying what she'll think if your body reacts in a normal way. Of course, you can always ask her, "Do you mind if I move my hips a little?" I can't imagine why she'd object. In general, a little communication goes a long way toward easing tensions – as well as getting exactly what kind of pleasure you'd like to get. As for ejaculation...well, unfortunately that comes with the territory, doesn't it? If you're not in a situation where you can take off your shirt, you can hold a tissue in your hand and put it in front of your penis when you're ready to ejaculate. This also makes cleanup much easier. Just have plenty of tissue handy in case of a spill or an especially large ejaculation (which often happens when you're being masturbated).