I use a set of Wahl hair clippers with the #2 or #3 guard (whatever length my head hair is at the time) to trim my beard. If it's been a minute, I might also do some general cleaning up around my neck or cheeks with a double edge razor.
Wahl makes a beard trimmer that's around $20 (
http://tinyurl.com/z3w4vql). Comes with a few different guard lengths so you can trim to whatever length you like, or take it off to get a little closer. I could probably get closer with the beard trimmer than I can with hair clippers, but I never take my facial hair down that much.