For my dress I used a pattern that sadly isn't available. There is a similar free shirt pattern on ICandy Handmade which you can use for the top and shorten it, and then add a flared skirt.
If you want to pin individual pictures from this page to remember the tutorial, please click over to the sites they came from. (Give them the credit for the hard work they did!)
Knit Patterns and Tutorials
Woven Patterns and Tutorials
Maternity Patterns and Tutorials
This one isn't specifically a maxi dress, but it would be easy to lengthen the skirt. And this style is so perfect for nursing. (I still wear my dress I made!)
Refashion Maxi Dress Tutorials