I don't sweeten many wines (I like mine dry), but for sweetening most of them I just make a simple syrup with sugar and enough water to dissolve it. I've also used honey (great in crabapple wine), but my favorite way is one you have to plan in advance.
Save some of the original must, stick it in the freezer, and then use that to sweeten the finished wine- that's the original definition of "back sweeten"- using the original must to sweeten the wine. It's great for wines like blackberry, since it retains more of the fruit flavor and doesn't dilute the wine with any water.