How can i get rid of a really bad tan line?

Tan more: Tan more without the same clothing to make the same tan line.

Don't go in sun: There is no fast way of getting rid of a tan line. That is just the body's reaction to being exposed to ultra violet radiation from the sun. The best thing to do is to stop exposing your self to more sun. Plus I would advise you don't tan your skin by sitting out in the sun or ever going into a tanning bed (horrible creations).

Tanning lotion: As Dr. Abdullah noted, avoid further sun/UV exposure (including tanning beds) so that your current tan can fade. In meantime, so that you're less self-conscious, try tanning lotions to even out your skin tone. Bottom line, only time can get rid of a really bad tan line.