Have the photographer stand on a stepstool or a small step ladder and shoot downward at you.
For a week or two before your photo session, learn how to do the yoga technique known as "nauli" and practice it every day before your photo session. This strengthens the ability of your abs to suck it all in.
There are a lot of different things you can do with your veil to hide your midriff. See this page on my web site for ideas:
http://www.shira.net/shirapix/gallery-shira-veil1.htm
Pose with your heels and backs of your legs pointing away from the camera, and twist your upper body to face toward the camera. It looks nice either with or without a veil. Like this:
