often times you just gotta go out even when your not feeling it.
try this if you feel like crap but have a ride planned: go out anyway on the condition that if you still don't feel like riding after 1/2 an hour you'll turn around and go home. that is what i do... guess what, i never go home.
in fact i often find my legs feel amazing after a while even when feeling like complete crap at the beginning of a ride (frequently on the second day following a previous days hard effort).
i do like riding mountainbikes because there is so much more to concentrate on other than the monotony of sore legs pumping up and down.