Okay, you have piles of clothes here. Cut it in half. Yes, you read that right. Your back will thank you later. You might want to consider getting a backpack, and keep your hands free.
Duffel bags can be great, but they can get very heavy.
If you are going to spending a lot of time with your bag, you might want to consider getting one with those handy-dandy wheels, and save yourself a lot of strain and sweat. The most popular is the rolling technique. Rolling your clothes saves space and reduces those pesky wrinkles. Then shove all the small stuff, like underwear, into the corners, or shoes. Put all your toiletries in a re-sealable plastic bag, or anything that can be securely closed.