When you lose weight, especially if you lose weight quickly, you risk getting saggy skin (just watch a finale of the Biggest Loser.. sad but true).
Instead of risking never being able to wear a sleeveless top (let alone a bikini) because of the excess skin, treat your skin right as you lose. Apply a good lotion (cocoa butter or shea butter is best) at least once a day (twice is better).
While you can't change nature (skin will obviously sag as you age), moisturized skin is pliable skin!
This works the same way that pregnant women apply cocoa butter to prevent stretch marks...