There's really no way to quickly get rid of it, since it slowly fades away in time. However, using an umbrella can prevent further tanning and can help make the tan fade. I don't think hats really help too much. If your town is currently set between mid-July and early September, definitely use an umbrella when it's sunny out. Playing at night, also helps tans to fade away.
Wide brimmed hats and long sleeve clothing, along with the cucumber/lemon pack helps with preventing tans. Like said above you can also carry around umbrellas. If you time travel, you can time travel backwards and that's how I get rid of my bad tans. ^^ I think it goes away really fast if you time travel backwards.
Wearing a hat would be a good idea.
Also having an umbrella is good.
When going to the island, make sure you have a hat on .
(As for hats that decrease a tan... I think a "straw boater" could get rid of a tan really fast...)