How to maximize the post-dried life expectancy of flowers?
If a flattened flower is okay for your purpose; you could adopt techniques from herbarium-sheet preparation. Keep the flowers in a fold of a diary or hardbound old notebook. Change their position every day (if flower is larger or jucier, you could use newspapers since entire papers should be changed at few to 6 hours interval at start days, latter intervals will increase gradually). At first day the flower (or anyother plant matter) would become soft and weak. Gradually it would become hardened like potato chips. Sunlight in way is very helpful. 

After completion, paste them with fevicol or such polymer glue in thin-amount ((since your work is permanent, feel free to fevicol or such. Botanists use  gum or glue instead so that specimen could be taken out later-on and dissected)), and some simple biology box tools like a pair of needles, a forceps and a scalpel with neat and clean, gently stretched way. (if don't have biology box, don't worry, other art tools and household things could be used). After completion of your artwork, dry it under hard sunlight. could take help of hair dryer. but never a iron. Any spray or coat is not used in a herbarium sheet, but if you wish to spray laquar or such; please make sure it is properly dried. Because if the water is locked it will ensure quick deteroration.

If you wish make it insect-proof; could use book-binding gum with copper sulfate... but that is toxic as well as gums and glues may take more time to get dried, so isn't preferable. If you wish, could wish clothings-naphthalene but then you have to keep it out of reach of children. 

But in a herbarium-sheet preparation technique; the flower-colour is not retained.

P.S. Herbarium sheet preparation technique already used by students for beautiful artworks especially with ferns, sedges etc. but rarely with flowers.