I haven't done it but was really interested at one point. I decided against it.
I'm sure you know this already but here are some things to consider...
Betta breeding is very tricky and rough on both the male and female. By making the slightest mistake you could lose both fish.
If sucessful you could end up with hundreds and hundreds of fry...what do you plan to do with all of them? If your fish are veiltails then people are unlikely to buy them.
How much room do you have? You'll need alot of tanks. A large one for the females (keep in mind that sometimes certain females won't get along which means you need even more aquariums), a couple smaller ones for the males, a small one for the fry, a grow-out tank (a 20-gallon is best), a couple breeding tanks (normally 10-gallons), and of course you need tanks for all the babies you keep.
I have a ton of great websites for you but I'm on vacation right now. I can post them when I get home tomorrow
. Or on Monday, depending on how sleepy I am
The best advise is to research a ton...especially about genetics and caring for the fry. And please don't just sell the babies to a petstore because who knows what'll happen to them then.