Livin, You can keep the same SIM card. I've had the same one for about 4 years now. Even when your credit expires, your service will stay active for at least a year. You just have to top it up before your next trip.
Check out this thread for some important Digicel codes that will tell you how much credit you have and when your service will expire:
http://negril.com/forum/showthread.p...=digicel+codes
Funny that you roam on T-Mobile. Mine roams on AT&T.