Tweezers! They keep fat fingers out of the way (mine anyway).
If you're gluing styrene to styrene, all you have to do is touch the styrene cement to one part, place it and hold it for a few seconds, and it will probably sit there. It really helps to turn what you're gluing onto so the surface is straight up. That way the part you're adding can sit the way gravity intended. I often use the long-nosed black bottle of Model Master to place glue exactly where I need it. If it's not that vital, the liquid seems to do the trick nicely. The liquid will also flow from behind (if there's a hole all the way through), which tends to protect your painted surfaces.
If you're gluing anything else to anything else, two-part epoxy can help hold the piece while it sets up. Or CA (super glue, cyanoacrylate if my mental spell check works) is another option. And with CA, you have "kicker" available which will make it set up immediately.