Updated: Sep 8, 2018
Any cycling clothes will do the job, add something to sleep in and casual clothes for when you are off the bike.
Be prepared for what mother nature throws at you, normally a cycling wind jacket with arm warmers should keep you warm. Don't let rain get you down as riding in the rain is an adventure on it's own.
Again the deciding factor for me is not to over pack on clothes and to select lightweight clothes that do not wrinkle easily.
Really think on how to maximize your clothing use. Fold and sort what you plan to take with you on tour making sure you have everything then roll each item into a small compact roll and secure with a rubber band.
A good idea might be to mark each item with a piece of paper. It just makes it easier to pick the item to unroll before wearing.
I find that most of what you need is what you are already wearing as part of your mountain bike outfit so only a few extra items are required for off the bike time.
What you might wear while riding:
Back Pack/Hydration Pack
Wind Jacket (cold weather)
Arm Warmers/UV protectors
Wrist Guards (optional)
Mountain Bike Shoes