I don't, I use panniers (or a large saddle bag for the one bike that doesn't have a rear rack).

But if you have to carry stuff off the bike a lot, panniers are not the optimal solution. Some panniers have optional carrying systems that transform them into backpacks. Here's Ortlieb's as an example. I don't have personal experience on how well they work. In my use, the biggest drawbacks would be not being able to carry both panniers, and having to deal with a wet and dirty pannier.

Have you considered strapping the backpack on the rack?

--J