If you don't have highway pegs give it a try... cost is very low.
I have a backrest as well on my 05 Venture and it does help on long rides but after everyone telling me to try the highway pegs I can go 1000 miles before the butt starts kicking.
What I do is keep changing it up every 30 min or so and I found that to work great for me.
The 10 min gas stops helps as well but the pegs will defiantly help you out on long rides.
I did post what I did to my 05 Venture, replaced everything with Alpine radio,Amp, remote control for radio but i guess someone didn't like the post and removed it.
I even had pictures showing how it was done.