Pretty much what was already said but especially recommend the liner. The liner will keep your bag cleaner and make it more comfortable since the liner will feel more like cotton as compared to the bag which will feel like...nylon. You really appreciate the liner on days you are not able to shower since the liner will feel a lot less sticky. I put mine in the stuff sack w/ my bag so it doesn't take up any more room. I really recommend a down bag as it will compress to a much smaller size compared to a synthetic and will last five times longer. At least that's the way it was years ago. Maybe synthetics are a lot better now. I also have been in many wet places but my bag has always stayed dry. This will help to keep it dry:
http://www.outdoorresearch.com/site/...y%20Sacks.html
Mummy bags are smaller, lighter and keep your feet warmer but for a lot of people, they are too constricting so put on some socks and go for the rectangular if you think you will sleep better.