I'm just getting started, but just mindfulness usually, awareness of something, like one's breath.
A while back, I was reading a web site, where someone wrote about a meditation of basically focusing on the mantra "I am."
Oh here it is:
AYP Home Page - Free Lessons in Meditation, Pranayama, Kundalini, Tantra
I don't know how effective it is. I might start trying it, though.