I have to go the cheapest route I can to make my T-comfy for a long trip, so I don't do all that much. I have a stock seat (which works better for me than the seats on my past 2 bikes, anyway) and the bike came with the sport handlebars- which helps me quite a bit with that slouching posture.
The three most import comfort factors I find are:
1) Windshield. (I use the National Cycle Plexi-fairing 3 for winter, and the steetshield for summer. Whatever you get make SURE its adjustable.)
2) Something to rest your back on, even if its just my duffel bag of clothes and my sleeping bag bungee'd to the back part of the seat. Heaven...and it doesn't take much.
3) Being able to move your feet/legs around at least a little. I kick down the passenger pegs every now and then just to move around a little even though I know its not the safest thing. I wish there was an easy way to mount highway pegs up front, but I haven't yet justified getting the engine guards from Ma's Cycle just to put pegs and lights on.
Storage is pretty important, so I did spring for a backrest/luggage rack from the dealer. Pricey, but despite the fact that the backrest is pretty low, you can crank down on the strap that goes UNDER the back pad and keep just about anything on the bike. Tie it down tight and keep the lightest stuff you can in it. I have an older Tour Master nylon sissybar bag with a roll bag that zips on and off the top.
Tank bags!!! I don't know if I'll ever ride without one. They're great- especially the ones with the map case on top. I can't imagine a long trip without one.
I have cortech saddlebags and I run the straps under the seat. It makes them a little harder to steal and stablizes them a little bit more.
So without any hugely expensive additions to my bike I've been able to go on trips up to 2000 miles (so far) without getting too uncomfy. Don't forget the tool kit, first aid kit and raingear!
Enjoy!
Chris