Fake tanner. Apply it in the areas that need to be tanned, and blend it out with some lotion around the tan lines. Practice before con and exfoliate it off if you can't get it to look even. You may have better luck with the gradual tanning lotions, but those don't get as dark, so if it's a huge difference, it may not work.
Otherwise, you can try those aerosol spray-on stockings. "Airbrush Legs" I think is one. It may not be opaque enough coverage to completely cover the tan lines, but it'll even things out more.