One of my assignores uses
http://www.usofficials.com which is very efficient and well organized. You can see who your partner will be, and look up all scheduled games, close out dates you are unavailable, and report info after games.
Another guy mails a post card which is barely legible, often with a baseball schedule on the reverse side for next spring. There are usually scratch-outs. This has been his only method for years, and somehow there are usually 2 officials at each game (or 3, or 4)! Go figure...