If you have a Google calendar, then perhaps you should be tweeting it.
Seriously — why not? You need Twitter fodder, and your calendar events are just sitting there, begging for publicity. Although no fully automated Twitter account can thrive (the lack of authentic personality really tends to turn people off), it's OK to automate certain aspects of your Twitter presence, such as upcoming events.
If you use Google Calendar, the bulk of your work is already done. All you have to do now is choose a method for automatically sending upcoming Google Calendar events. Here are a few options to try:
Make sure your Google calendar is public, then paste the XML feed URL into your Twitterfeed account. The blog Maria's Guides
gives a great explanation from creating the accounts to naming your calendar feed.
This service will automatically search for and post events to your Twitter account from your Google calendar every hour.
This service requires you to follow it on Twitter, but all you have to do is sync your accounts, then send Twittercal a direct message @gcal.
If This Then That.
ifttt.com allows you to create "recipes" that automate actions between your various online accounts. You can create a recipe that specifies events from your Google Calendar to be automatically fed to your Twitter account.
Do you recommend any other services?