I don't modify swimwear, mostly because I don't wear it. I would imagine that you'd have to get similar material to the bikini's original material, though.
No matter what size I've ever been, I think I look terrible in swimwear. All I see is fat when I put one on, no matter what size a really am.
I did find this for thread and needle recommendations.
http://www.ehow.com/how_2190969_alter-beach-gear.html