Also Don not sure if your aware but there are a few plants whose dye properties stay even if the plant is dried, Not many though. Some of em for certain dye colors you even want to dry em first.
Other than that I agreed with the others - add in alcohol, or toss them in the freezer.
A vacuum sealer also works from time to time, but isn't your best bet. I've had numerous berry dyes grow mold even after vacuum sealing when I was younger.
Adding alcohol, be careful though - too much could damage whatever your dying, and any amount is going to dilute your color so some degree. Most plant dyes have subdued colors anyway, so not a raw deal, but sometimes a deciding factor on dyes that already very light, as adding in enough to preserve it could make it too light for what your trying to achieve.
There many many books on the subject, and if you can't find what your looking for from the good folks here or on the net, I suggest your local library, ours had heaps of em on it.
