are all yer pullies in line?? is you overtightening them? are you buying cheap belts? ave you got the right size belt? do i get a prize if i can think of 5 answers? ave you got a rough bit on one of yer pullies?
How about increased loading on the belt?
Ie. The 2Kw stereo you may have fitted or the rally spot lights?
or doggy water pump or fan.
Its not easy to check the loading on the alternator / dynamo without a ameter