Sorry, but if you can't manage a nap, there's no real way to LD during the daytime. You can always prepare for LDs during the day by reality checking and planning your goals, reciting mantras, etc., but as JoannaB said, you have to sleep to dream. Here's what I'd suggest: keep practicing LDing during the day so that you can have them at night, and if you ever do feel tired enough to fall asleep during the daytime, take advantage of that. But don't try to force yourself to sleep during the daytime when you're not tired or you'll just end up wasting your time.