Hello!
It's quite nice to see a new person interested in Star Wars autograph collecting! Unfortunately, this forum isn't quite as busy as it used to be several years ago. Most of the address listings on this site are at least a couple years old, and some are outdated (like the two return to senders you had.)
Hopefully I don't offend the very few people that actually make the occasional post here every couple of months, but if you would really like to get into the hobby, I recommend two alternatives. You probably wouldn't have gotten a reply for quite awhile had I not coincidentally decided to check in on this forum today.
Firstly, I actually just created a site of my own last week that is dedicated to Star Wars autograph collecting. At the site, you will find an address database with well over a hundred listings for people involved with Star Wars and hundreds more to come. I hope you will consider joining. I would be more than happy to help you with questions about the hobby in the forums on my site.
http://starwarsttm.ucoz.com/
The second resource is the forums on a site called Rebelscum. In the autograph collecting forum of that website, there is a great deal of information posted on a daily basis by folks like myself who have been collecting for many years. You might find the information there a good resource.
http://forum.rebelscum.com/forum.php
I will give you a bit of important advice right off the bat, and tell you that most of the "bigger" cast members from the movies (ie. Natalie Portman, Liam Neeson, Samuel L. Jackson, Mark Hamill, Harrison Ford, etc.) simply do not sign autographs through the mail. Most of their autographs go for over $100. I know it can sound a little discouraging at first, but trust me when I say that it is still a very rewarding and fun hobby once you slightly lower your expectations.
If you are interested in knowing more, feel free to keep in touch with me on my website via either the forums or contact page. I can't say how often I'll be checking in on these forums.
Best to you!
Zack