Twitterfeed at http://www.twitterfeed.com is also a very handy tool. It is far more secure to log into than the other sites I have mentioned as it uses Open ID which is a bit of a pain. However, it is worth persisting with in the absence of an alternative tool.

What Twitterfeed does is that you can set up your blog to feed directly via RSS to your Twitter account.  So every time you make a blog post, it gets tweeted automatically on your behalf.  You can use this in conjunction with Tweetlater with the first post coming through Twitterfeed and then subsequent ones at times you specify via Tweetlater.

Technorati Tags: RSS to your Twitter accoun, Twitterfeed