I second everything Kaydreamer just said, and I add two things:
Motivation: This is a hard one sometimes, but I have two things I use to get myself excited about a project again.
1) I go back to what got me excited about it in the first place. Do you like to draw fight scenes? Expressions? Write snarky dialogue? Re-read your favorite portions and soak them in, enjoy them.
and 2) If you had a luster and lost it, go back to WHERE your project seemed to lose its shine and figure out why it stopped working. If possible, re-work that section so it DOES excite you, and let the renewed inspiration carry you from there.
Manageability: Yes, have a routine, do a bit every day. Yes, have mini-deadlines to help keep yourself on track. Yes, pre-write your script so you know WHAT you're going to draw next. But make. Sure. It's. Manageable. And what I mean by that is, don't work yourself to the bone by trying to do too much in one day, or making your deadlines too short. You should not be in pain after you quit drawing for the day. You should have time to rest, take care of yourself, and do something else. Art is an outpouring; you need time to put energy and motivation back IN you. If you go too hard for too long, you WILL hurt yourself or burn out, and then you're back to missing updates or hiatuses while you get yourself back on track. Comics are a marathon, not a sprint (unless you're doing a short-run contest or something...), so pace yourself. Future you will appreciate it, and your readers will too.