I've done a lot of touring, both short trips and longer ones (9,000 in 18 days etc) and have almost got the packing list down to a fine art.
First you need to make a list of things that you cannot travel without.
1 Passport/Driving Licence
2 bank/credit card
3 insurance/bike docs if on own bike
4 toothbrush
5 Riding gear- ATGATT
6 Firearms licences plus firearm in appropriate countries
7 Adventure first aid kit. Include day/night nurse, anti-inflammatories and paracetomol
8 GPS
The rest is dependant on what you feel comfortable with
My list is as follows:
9 Shorts (one pair)
10 T Shirts x 2 for each week away (always end up collecting additional T Shirts on bike trips!
11 comfortable shoes/sandals for when off the bike
12 additional toileteries in small containers
13 non-riding trousers (one pair)
14 change of underwear for every day I'm away (if riding in countries where I have connections, the amount can be less as friends washing machine can be available)