There are several free tools that allow you to schedule your Tweets. I recently signed up for Tweet Later and gave it a test drive. The design and mish-mash of fonts can make using the dashboard a little confusing, but the features are great.

Once you add your Twitter account you can opt for the following:
You can send an auto welcome message
Auto-follow anyone that follows you (or manually approve before following)
Email digest of replies to your tweets.

Scheduling Tweets:
Type your tweet (announce new menus, hours, special promotions, show dates, etc.)
Save as a draft or schedule to publish (immediately, minutes/hours/days/weeks from now, or a specific day and time).
I scheduled a new featured link to post every Friday at 9am. Follow 15minutesmedia on Twitter to get our tweets.

There are a number of additional features you can unlock with a paid subscription.

Pretty cool tool. Now I can sit down when I have an hour or two and schedule a week or two's worth of Tweets.

Want a more detailed breakdown of features? Read this blog post: http://www.squidoo.com/scheduletweets