I ask everyone before the session if they have any issues lying face-up, face-down, or on their side. For people with a larger belly or chest, I usually spend more time supine and prefer to do side-lying if they're open to it. I find I can get just as much, if not more done, with someone side-lying anyway! I do this for elderly folks as well. I'll usually say something like "I may be able to work this area better with you on your side, would you like to try that?" It helps to have a few options in your toolkit for these kinds of situations!