I've never had a ton of trouble motivating to stay in at least decent shape. Not that I'm some kind of triathlete champion or ultra-ripped gym rat or anything. I just feel so crappy if I skip a whole week that I'll get out and exercise the next week.
One thing that really helps me is to try to get as much exercise through fun activities as possible (instead of through straight exercise like running, lifting, etc.). Through the years I've gotten my exercise through obsessive surfing, hand-balancing classes, partner acrobatics classes with my wife, yoga classes, a trail running phase, regular intense tennis sessions with a good buddy... You get the idea.
If you play, say, two physically demanding sports, each once a week (and you push yourself, being the chubber who doesn't play defense and just sits around on offense and heaves up threes every once in a while doesn't count!), you only need to find one other activity a week that gives you great exercise and you're on a sustainable 3x a week path. Toss in a P90X style strength workout and you're in great shape with only one "workout" each week!