A covered terrarium is an excellent way to build up your sphagnum supplies if you're having trouble growing it outdoors. If you're using the tray system, and using something like an aquarium with a plexiglas or saran-wrap lid, the humidity should be sufficient to keep the sphagnum from drying out (until it starts growing out past the edges of your pots).
You can either spread it around in little pieces throughout your plant pots, or give it it's own. As NaN mentioned, it tends to grow better at first if it's clumped up a bit.
If you don't have a terrarium, you can also try a clear, covered bin with some holes poked in it. Put it somewhere that will get bright light, but out of direct sun. Put half-an-inch or more of your regular potting mixture into the bottom and "seed" it with sphagnum strands.
(You could possibly give it some direct sun, I'd just be worried about it heating up too much)
