After casting around for a good Twitter client for the desktop I have settled hard on Twitzap.
- updates automatically (and you can set how often it does this)
- allows tweets even when Twitter is down
- allows you to set up channels really easily to follow one person or a group or a search term / hashtag
Another service I really like is twittertise. Ostensibly an advertising client (gulp) it does allow you to 'pre-tweet'.
As I've noticed that people can miss tweets (I do), it's useful if you need to throw out a question or promo something to be able to set up a retweet over a few days or at different times. It's obviously a tool for abuse as well but like most things it has it's angel side as well it's demon one :]
Mashable's list of tools for managing multiple accounts.