Being a workaholic helps. XD
But to be serious. I find it easier to keep going if I work on the comic chapter by chapter. Finishing the storyboards/sketches for an entire chapter in one go, and then going back and alternating between inking and colouring helps keep me on track. Also, having scenes/chapters/characters I really want to draw in later chapters helps me keep my steam up, because my readers need to read/know all the stuff that comes before that if they're to enjoy it.
Grassblades updates twice a week, but I've gotten to a point where I have a.) a big enough buffer that I don't need to draw this week's update right before I update, and b.) am familiar enough with characters/setting that the drawing is pretty quick and painless at this point, so the actual work doesn't take up my entire week.
It also helps to have sideprojects going on - shorter comics or illustrations I can switch to and work on whenever I feel like it.