Best way to grow or keep your sphagnum live is to plant it in a pot that is 3/4 full of wet peat moss. Press the live
sphag into the peat moss. Then find a water tight container larger than the pot, like a large plastic potato salad jar, drop the planted pot into it and then fill the container with distilled or r/o water up to the top of the live moss. Let the water evarporate back to the surface of the peat moss before either spraying with a mister, using distilled or r/o, or adding more water to barely cover the live moss. Keep in a partly shaded spot but be sure the plants get direct morning sun or late afternoon sun. Keep the plants in a cool spot.
Check out my bog journals below.

On the flushing out of the fertilizers or salts-THIS REALLY SOUNDS LIKE ADVICE FOR GROWING ORCHIDS!!!! This is not a workable procedure for plants potted in peat moss!
Best approach is to use either distilled water or reverse osmosis water PERIOD.

Here is a link that might be useful: sphagnum bog in a tub

Carnivoor,
Q1: Just keep the sphagnum moist and it will be fine on its own, you don't need any other medium for it to grow on.

Q2: Yep, if things just drain into a drainage tray and get sucked up again into the growing medium, then you aren't flushing anything out. Toss the water in the tray occasionally and you do get rid of any salts. But I think terrestrial man said it best... use distilled water and you avoid the problem entirely.
-Don

Thanks for the answers.

I've read that they should be on a layer of dead sphagnum / peat. So that is what I do. I keep saturated at all times and under artificial lighting.

i place sphagnum moss in a water tight seed tray and then cover it with water (distilled or tap with some viniger in it)

No need for vinegar. It is already acidic.