Here is a link to the firearms policies in Canada along with the forms you can download. www.cfc-ccaf.gc.ca/faq/default_e.asp
Your outfitter should send you one when you book your hunt. It's about $40 US at the border when you submit the form.. You should stop on the US side and get a Certificate of origination for your firearm as well. This shows where your firearm originated. Meaning you didn't get it in Canada or elsewhere and it is from the US. It's free and I didn't have one when I went to Canada and on my way back to the US side, US Border Patrol said they could confiscate my rifle. It takes a minute to fill out.
Make sure you get an FDA form for bringing back your meat to the US. Due to the Mad Cow scare you will need one and your Outfitter should give you one before you leave camp with your meat. The only other thing is make sure you keep the tag on your animal when coming back to the US. My outfitter said I didn't need it and kept the tag. At the border Fish and Game said I should have had one but were pretty cool and filled out a form to let me bring it back into the US.
All this paper work doesn't take long and it's really not a hassle. The other tip I could give you is to take 20's with you instead 100's. Some people in Canada will not give you an exchange rate for 100's and it's almost a $20 differance.