Shop knowing your girlfriend's clothing size. While swimsuit brands have their own size charts, generally speaking a girl who wears a size 0 to 6 clothing is a small, 8 to 12 is a medium and 14 to 16 is a large. There are also petite and plus-size suits as well. Talk to the salesperson and get the layout of the store before shopping.

Know your girlfriend's personal style before you shop. A girl who bares skin easily might readily go for a thong or G-string bikini, while someone with more modest tastes would be happy in board shorts and a tankini top. A bandeau or triangle top with a little center knot creates a nice bust line for smaller-busted girls; a girl with a bigger cup size could use a heavy underwire in a halter or off-the-shoulder top. Just make sure to balance the top with bottoms featuring some flirty details like ruffles or ruching. In addition, high-cut bikinis elongate the shorter gal's legs, while a bikini band helps hide muffin top.

A fun pop of color and a playful pattern can turn a blah suit beautiful. If your girlfriend is bottom heavy, choose a solid candy-colored bottom without details or extra fabric. This shifts the eyes upward to her top and face. Slimmer gals look good in bold geometric patterns, graphics, paisleys and florals. Keep in mind things like color-blocking, which is a useful way to break up a long torso, and neon colors, which complement tan or dark-skinned gals.

Refer to your girlfriend's body type to figure out where the swimsuit detailing should be. Smaller-chested girls look good with ruching, layered fabric and fringe that help give the bust area a lift, while bigger-busted gals can go for cleaner, more streamlined lines on top. Any size girl can wear fun embellishments such as cute cut-outs on the bottom, jewels encrusted on the top and interesting embroidery. Look for swimsuit detailing like the ones she likes on her clothing, such as metallic accents, studs or little ties and bows.