Be careful with one piece swimsuits. I am long in the torso for a girl my size (5' 7.5", 160 lbs) and very few of them fit me. Fortunately I don't look bad in a bikini and on the five or so occasions I have worn one in public I have never had a problem.
1. Look for swimsuit and bikini bottoms that are wide in the crotch. Even boyshorts are no help if there is not enough crotch width and can leave, um, boy bits falling down the leg opening.
2. Interestingly enough, patterned bikini bottoms seem to work better at hiding a tuck than solid ones do.
3. Tops with ruffles are a flat chested girl's best friend. There are also lots of suits with pads and push up devices in the tops.
4. Don't be afraid to buy two different suits to get the right top and bottom. I bought a solid color Michael Kors bikini where the padded halter top looked great but the bottom showed my tuck no matter how hard I tried. I paired that top with a bottom from Inmocean that has thick fabric, a wide crotch, and a pattern that hides the contours of my crotch to the point that I can tuck into it without taping. I've found that bottoms from Raisins are good that way too.
3. Always dress rehearse in front of a mirror.
4. I will do a tuck and tape (using medical tape for sensitive skin) for wearing smaller bikini bottoms in front of others, but I do not recommend it. Even though a year plus of hormones has made tucking easier, it is still uncomfortable for me. Swimming is for fun, and I like to be comfortable when swimming.
