Change. I get bored with the same old, same old. So I change routines ("what's this going to do for me?") or change focus altogether (right now, I'm planning another whatthehell BB competition entry. But before that, I trained for road races -- I was working up to a distance trail race, but hurt myself -- and wilderness races.)
If it stops being at least minimally fun (in the longer view), or if I find myself thinking of the walk downstairs to the gym with dread and apprehension -- well, time for a break.
I also found, incidentally, that "day in, day out" was a major recipe for burnout and workout loathing. Back when I was into that, I was doing it solely out of desperation and self-punishment anyway. Rest days are as important to my motivation as the workout days. Nothing sucks the life out of me faster than dragging ass with exhaustion and more or less twingy muscles 24-7. After a day of rest, it's the good feeling of needing to work out.
