One key is to have a nice, flat surface behind the mounting point for the wing. If your body material, hackle, etc. form a shoulder behind the mount point, it will push your wings upward.
Do verything you can to minimize bulk at and just behind your tie-in. This include minimizing wraps of hackle stem, tinsel, etc. Tie off materials under the hook shank, not on top.
Some of the things you can do to avoid this include tapering the body as you arrpoach the wing tie-in, using your scissors to trim materials just behind the tie-in point, and using a flat-jawed plier to flatten the area just behind the tie-in point.
Also, make your rearward wraps slightly less snug than your forward wraps; a snug wrap over a soft undersurface will tend to bend the wing upwards.