I think about things that calm me down, like how my boyfriend loves me. I also think about the facts that directly contradict whtever it is that is worrying me. I focus on those facts especially when the fears start yelling in my brain the few nearly impossible sets of events necessary to cause the worst possible situation. I just focus on solid facts that prove i'm safe, etc.
Also, if i'm already freaking out, singing a set of lullabies, singing the alphabet, counting, or singing as many songs as I know in my head can often help. A last thing that helps is to imagine myself in fantasy situations, like an episode on star trek, dressed up hanging out at conventions, etc. Situations that haven't hapened yet, or (like the star trek episode) can't happen. These things help me fall asleep at night, and relax during the day.
If i'm at work, then I can easily distract myself with whatever it is i'm doing; and during the day I can normally distract myself with things.