When I jack off in bed, I usually just lie on my back, maybe sit up a little, and shoot over my stomach and chest. If I'm really worked up and have a lot of cum, then I try not to let it run down my sides by rubbing it around or catching it with my hands. And if I didn't think ahead to have tissues or a towel nearby, I just go to the bathroom to clean up and then go back to bed. Seems simple to me. Of course, if you have to leave the bedroom to get to the bathroom, then you should just try to have an old sock, tissues, or a towel handy so you don't have to try getting to the bathroom and risk other people in the house seeing you running around dripping cum.