Do you mean making them healthy, or preserving the physical flowers themselves?
If you want to preserve the physical flowers, the tried and true method of dry
pressing is best. Just place them on dry newspaper, press them down between
two big books, or use some type of flower press (they sell them at those nature
stores springing up in all of the malls nowadays). Sandwich each layer of
flowers to be pressed between 2-3 layers of newspaper (black and white only),
and a layer of cardboard on each side (between the flowers and the books too).
Use corregated cardboard, and place the press arangement in front of a fan in a
dry location. Switch out the paper every day or so, and the flowers will even
keep their color if not exposed to sunlight. It even works better if you have
a 130 degree F convection oven (what we use at the university for our herbarium
samples). Hope this helps.