For focusing on stars, I suggest using a Bahtinov mask, which uses purposely-created diffraction spikes to determine correct focus.
Bahtinov mask by Justin Dolske, from Flickr. CC BY-SA-2.0
This image montage is an example of a Bahtinov mask on a telescope focusing on the star Betelguese. The center image is correctly focused; the other two images are under- and over-focused, respectively.
There are several manufacturers of Bahtinov masks, or you can make your own (there are even online calculators that will generate Bahtinov mask patterns to print out and cut). The size and spacing of their diffraction gratings is dependent upon focal length and absolute aperture.
Personally, I use the SharpStar2 4x4 filter from LonelySpeck (I'm already a 4x4 filter user). I couldn't be happier with it.
Agena Astrophotography makes a mask that snaps into any 77mm UV (or clear) filter. Agena's mask is very inexpensive (about $15), and a cheap UV filter can be had for under $5.
I have found that the narrower/tighter slit pattern on the SharpStar2 filter creates larger, cleaner diffraction patterns than the wider-spaced grid on filters such as Agena's, which makes focusing easier. Regardless, even using patterns like on the Agena, Bahtinov mask focusing is so much easier than without using a mask at all.