Well, I used to have more focused mantras, and now I'm trying to utilize them more to work on focusing.
Probably my most used mantra is to tell myself to "just breathe, center." over and over.
"This too shall pass," is a common one with the kids ;-)
When I'm meditating, I'll use a word that focuses on what I need to do...like "focus" or "I feel healthy" or "peace." I'll also use the some standards, such as "om," "hari om," or "om shanti." I only know these through yoga practice, but in Sanskrit, "Om" is the basic sound vibration of the universe that vibrates in every cell of the body. "Hari" is an aspect of God or the Spirit, whichever you prefer, which removes obstacles, and "shanti" means peace or the true nature of Spirit.
My grandfather and my dad say the Rosary, which I also consider a mantra. It's a steadying force/word, the repetition of helps realign the mind and body towards the word(s) and meaning within.