Since my meditation tends to be Anapana, or Vipassana, I don't have to worry too much about losing focus, as that's part of the process. That being sad, I often use Anapana as a focus before large rituals, or more intense meditation.
So what do I do to focus? Anapana, or simply put, I focus on the sensation of my breath. As you advance in the practice you focus on smaller and smaller areas, but when you're just starting you tend to focus on everything within the triangle from the top of your nose, to the corners of your mouth, and just feel everything, external and internal, as you breath. You'll lose focus...a lot...but keep bringing your attention back, it helps develop focus and awareness.
When we first begin all things simply are.
As we grow all things are external.
As we learn all things are internal.
As we understand all things are not.