I've never used a service, but even if you do, I recommend "How to Form Your Own California Corporation" (http://www.nolo.com/products/how-to-form-your-own-california...) from Nolo for a good overview of the process if you're incorporating in California. It's not really very hard to do all the required filings yourself.

If you're planning to look for venture funding, you need guidance beyond what's covered in this book, but it's still a good intro.