* Use cotton tampons.
Look for organic cotton brands such as Cottons, Natracare, Maxim, Seventh Generation, Lola, Emerita, Toms, Veeda, etc. and AVOID rayon brands such as Tampax, Playtex, Kotex, OB, and stores own brands. The reason is that rayon is rougher and more aggressively absorbent, lose more fibers, and they are chemically treated which may impact on TSS risk as well as having other health risks.
* Use lowest absorbency.
General rule of thumb is to start with light absorbency: if it leaks within 1-2 hours use the next absorbency up, but if not saturated with blood after 4-6 hours then you need to use lower absorbency or if already using lowest absorbency then switch back down to pads. Time it so you can insert a fresh tampon just before going in the water, make sure to insert high enough, and check every few hours.
Absorbency is a factor as tampons absorb vaginal moisture causing small tears in the vaginal walls - think of it a little like lips of your mouth getting cracked when dry - this allows bacteria and bacterial toxins into the bloodstream, if you lack immunity this can lead to TSS. Using a higher absorbency is an increased risk, but primarily this is when your flow isn't heavy enough - without sufficient menstrual flow the higher absorbency tampon will absorb more vaginal moisture so thus cause more damage.
TSS is a low risk but still one you need to be aware of and take precautions to reduce risk - TSS is also not the only risk from tampon use. You ARE Loved (TSS information) http://www.you-are-loved.org
If you don't want to get TSS, have you considered menstrual cups or softcups?
Cups are internal like tampons but far safer and more hygienic so don't pose the risks of TSS or other health risks that come with tampon use, this also makes cups more convenient as they can be worn for up to 12 hours with any flow. Menstrual cups are especially good with heavy flow and when active as they hold far more flow than tampons, have a seal to prevent leaking, and can even reduce flow for some.
What Are Menstrual Cups? - http://www.scarleteen.com/article/body/what_are_menstrual_cups