Depends a lot on what your ceiling is made of. Generally, you want dissimilar material (absorptive & reflective).
Carpeting over concrete, with a thick pad is very popular. You can ask many people at these web sites (Ethan Winer & others):
http://forum.studiotips.com/
http://www.recording.org/
http://www.audioasylum.com/forums/rives/bbs.html
http://www.bobhodas.com/publications.html
http://www.ethanwiner.com/default.asp
http://www.realtraps.com/
Good luck.
Carpeting over concrete, with a thick pad is very popular. You can ask many people at these web sites (Ethan Winer & others):
http://forum.studiotips.com/
http://www.recording.org/
http://www.audioasylum.com/forums/rives/bbs.html
http://www.bobhodas.com/publications.html
http://www.ethanwiner.com/default.asp
http://www.realtraps.com/
Good luck.