Quickly inserting pins into the thorax will help prevent shrivelling. Once the mantis dies, be sure to immediately put it in an environment completely devoid of moisture, in this environment (preferably your foam mounting board) apply one pin for the thorax (middle section). If the mounting environment is clean and dry, the mantis should be preserved with minimal color lose (preservative liquids increase color loss). I find extending the wings with pins makes the drying faster, and try not to puncture the soft parts of the body (i.e. eyes, or abdomen) as this can cause a nasty leakage which will attract things like flies. I use small pins to brace the joints of the mantis' appendages (don't puncture the exoskeleton with the brace pins, simply put them where the joint is to retain your desired pose). Here are some links to nicely preserved specimens:
http://homepage3.nifty.com/mantis/img/specimen/metallyticus_s.jpg
http://users.pandora.be/thebugmaniac/alainimages/other insects/mantidae/hymenopus_coronatus f.jpg
The second link displays a book 'pressed' specimen (the specimen was pinned, abdomen punctured to release juices, and then pressed lightly with a small book). Though this does cause shrivelling, if done in a clean and in a moth-free and fly-free place it can yield the best poses. Hope that helps.