Get the Right Size
The first thing on the list is getting the right size. Don't buy a bathing suit that is too small. Besides making our bodies looked squished, the extra pull on the seams will cause the stitching to break down quicker than a suit made for your size. No one but you knows if you had to go up a size to not stress the seams. The number is for reference, it doesn't define who you are. And it will be that much easier to get in and out of.
Don't Over Stretch
When you are putting on your bathing suit be careful. Don't yank and pull at as you are pulling it on. People tend to think swimwear is indestructible but it isn't. Put it on like you were slipping into a delicate negligee.
Wash Before Wearing
This is more of a caution for you than for the suit. You don't know who has tried it on and what they were wearing. Not to mention that it has been sitting some where collecting dust. Washing also removes any contaminants it might have been in contact with during manufacturing and shipping.
Rinse Immediately After Swimming
Most people don't wash their swimsuit every time they wear it, especially if it is on a daily basis. It doesn't matter if you were swimming in salt water or chlorinated water your swimsuit needs to be rinsed. The salt and chlorine can fade the fabric and loosen the fibers over time. Rinsing the swimsuit with tap water instead of letting it air dry after swimming will prolong its life.
The surest way to prolong the life of any swimsuit is to hand wash. It only takes two minutes. Use a delicate wash liquid. You can be drying off and getting dressed while it's soaking in the water. Give the important parts and gentle rub and squeeze the soapy water through the rest. Rinse soap out thoroughly, ring out excess water (don't twist the life out of it) and your done.
Lie Flat to Dry
You know those net drying racks we use to dry our delicate sweaters? Well use them to dry your swimsuits too. No hanging them over the shower head in the bathroom or drying them on the deck in the sun. They are delicate, not sling shots.
Stock Up on Extras
If you can, take advantage of sales so you can always have a swimsuit on hand when one is dirty or drying. If you live in your swimsuit during the summer months you should have more than two. End of season sales are a great time to look for back up swimsuits.
Most importantly, get out there and enjoy your new swimsuit. Don't forget the sunscreen!