When touring, I have generally have two front and two rear panniers with a top bag across the top of the rear rack. In total my panniers and trailer are rated to carry 78 kilogram (172 lb.) but excluding water, I would try to only carry about half this weight with it. Add in 15 litres of water and it is still a reasonable load to be dragging around the country. Through in a rough road and all this weight can create quite a sheer factor that can snap mounting connections.
2 x *Ortlieb Front Roller Pannier bags
2 x Ortlieb Rear Roller Pannier bags
1 x Ortleib Ultimate 6 Handlebar Bag
1 x *Ortleib Top bag
When distributing the weight, I like to keep the front panniers as light as possible with most of the weight spread between the rear panniers and trailer. Given that I always dismount to the left of the bike, anything I need in a hurray or often, is packed at the top of the bags on that side of the bike but without having all the weight on one side either.
A typical pack would be as follows:
Rain Jacket or jumper
First aid kit
Spare water bottle
Top of rear panniers:
Bike repair kit
I can guarantee that I will accumulate stuff as the journey progresses so I make sure that there is empty space at the start that can be filled along the way.
If taking the trailer for extended trips or where extra food is carried, the gear from the top of rear panniers is moved to the trailer and the extra food replaces it. Most of the gear stored here will fit inside of the cargo bag. For stability, the heaviest gear is stored low and toward the front of the trailer. Flat gear like tarps and spare trailer tire can be secured on the trailer below the trailer bag.
The page Bike Gear has full list of the bike storage gear items I use for a ride.