There are several apps out there that let you manage and schedule your tweets. The one I'm using right now is:
It lets you manage up to 5 different tweeter accounts for free, and schedule you tweets. Because it is web based, you won't have to connect to the internet for the scheduled tweets to be sent. Cool! It's simple, easy to use, and works great for organization that have multiple Twitter users. These users would simply log in on separate Cotweet accounts, and can be assigned tasks to respond to Twitter messages.
Here's how it looks like: