Well I guess I have a couple....

My motto for mountain biking is "Black side down, blood on the inside!"

I have learnt a few good mantras from the guys I ride mountain bikes with though...

HTFU (as in Harden The F*** Up) and I wear one of those little rubber wristbands with this on it when I race. Sometimes I look at the wristband and say it to myself up really big hills.

TUP (as in Toughen Up Princess) - a little less effective!

But back in wholesome "Mother mode" there's just about nothing that beats singing "one little, two little, three little indians...." or "10 little ducks went out one day..." They've helped me through a few half marathons but I don't use them so much on the bike - maybe I should! And you can always add to the number of ducks that went out for those really long hills!