John
What do you want to use the preserved plants for? Modelling or decoration?? As there may be different requirements.
I haven't preserved any plants myself as in drying or placing in glycerine (I think it is) but I am sure that a search or things here either in these forums covering dio bases and vegetation or in the features will have something that mentions preserving techniques.
The closest that I have come to "preserving" anything has been using dried herbs as foliage on large trees and then spraying light coats of different coloured paints onto them to get the shades I wanted. Also the odd dead twigs twigs I have recovered from the garden have just been painted whatever colour and then added to dios without trying any particular preserving techniques. So far these herbs and twigs are still in good condition (4+ years later) so I haven't really applied my mind to any better ways to keep them.
Brian