In the Store: If you sleep cold (you know who you are, Popsicle), look at bags rated at least ten degrees below what you think you'll need.
Climb in, roll around, stretch out, cinch the hood. If you're not comfortable in the store, you won't be in camp.
In the Field: Air your bag out each morning to dry any moisture.
If you're base-camping for several days, leave your bag unstuffed.
Need a few degrees more warmth? Add a silk or synthetic bag liner, which also serves to keep your bag clean.
At Home: Air your sack (again) to ensure it's completely dry.
Always stow your bag in a large, breathable sack.
Don't fear the washing machine. Launder according to manufacturer instructions.