There are a couple of reasons tampons leak: either you are using one that isn't the right fit for your vagina and your period is leaking around it, or you are flowing fast enough that the tampon is getting full before you change it. The best way to prevent leaks is to find a tampon that expands to fit you as it fills up and to change your tampon every few hours. Experiment with different tampons and see which one works best for you.
Well, this happens to me a lot. Just so you know, I try to gauge my flow and pick out the right size tampon, but guess what, leakage happens. So listen, here is my trick: the handy dandy panty liner is my friend. Sounds like you are tampon-challenged like me, so I suggest, if it is leaking in the beginning, maybe going up a notch in absorbency for those heavier days. The other idea is to try changing your tampon more often during your heavier days. Remember my trick, the panty liner; some of my friends are tampon champs and don't need them. Happy days for them; I just know what works for me!
Leakage can definitely be a bit embarrassing. I know I've had my fair share of mishaps where I've needed to throw a sweater around my waist. Are you changing your tampon frequently? And by frequently, I mean every four to six hours. Each day of your period can be different, so I use a higher absorbency for those heavier days. Make sure you always have a spare pad or tampon in your locker or purse so you're always prepared and never have to face those embarrassing moments.