I guess the serenity prayer. I carry it on a card in my purse, have it pinned to my PC at work and I have SERENITY tattooed on my arm.
I think this is mine too, although I tend to paraphrase it when I'm trying to figure out whether I'm right to be stressed or not.
I also love the desiderata - although it's a wee bit long to be considered a moto.
Love love love both of these. Serenity prayer was my first thought when I read this topic and desiderata has gotten me through some tricky times too.
When I was at uni there was a famous ladies loo in the union with a door that was covered on the inside with inspiring messages. My favourite one was "everything will be alright in the end. If it's not alright, it's not the end".
Also have a mantra of "all is well, all will be well" which brings my mind nicely back to centre if I start worrying about something.