I've kept Bettas since I was a teenager. I'm now in my mid 30s. There have been small gaps that I've not had one, but for the most part, I've had at-least one. You'll see me brag on here (and I think, rightfully so!) that I have never "killed" a Betta. I've never even had a sick Betta.
I think there are several variables that have lead to this:
1.) I don't buy sick Bettas to begin with. All my Bettas have come from a locally owned pet store that generally buys all of their animals from hobbyist breeders and the like. Asides from the big aquarium fish, even most of there puppies are brought in from owners that are like, "Oops. We didn't fix our dog. Take our puppies?" It's small, the staff are knowledgeable and they aren't overwhelmed by so much inventory.
I always make sure that the Bettas I buy are in good health. Their fines are nice and not torn, they have nothing on their scales that could potentially be a problem and they act healthy. They react to me when I approach their bowls in a relatively normal manner. They swim and flare, but don't dart around and freak-out or do nothing at-all.
2.) I never ever overfeed. I will spend time with each of my guys, feeding him one pellet at a time until he looses interest. If the last pellet doesn't get eaten, I remove it. There is absolutely nothing in that tank to rot beside the fish's own feces.
3.) I use live plants. I'm a naturalist threw and threw. You can thank my high school botany/zoo teacher for this. I believe that if you are going to try to give an animal a home, it should be as close to its natural habitat as possible and, I am sorry, silk and plastic plants do not exist naturally in the waters of Thailand.

Not only this, but it is an awesome alternative to a filter for my set-ups.
4.) I don't freak-out. I keep them warm, but I'm not checking their water temperature constantly. I change their water often. It's dechlorinated, but I don't freak-out about the PH and so on. I figure, my guys are blowing bubbles it is good.

Mind you, I had a rather listless fish last-week. I did check the water. Just to rule out potential causes. (He was cold. That was all.

)
I'm going through all of this, because I come to this forum (Which I love! Don't get me wrong! There is all kinds of useful info and the people seem lovely) and I have to wonder if there aren't a number of people that make this seem much more difficult than it actually is?
Seriously, I wanted to freak-out the first day. "I'm doing it all wrong!" But my long-lived, healthy guys tell me otherwise. And I wonder if a lot of people aren't running into problems on here for the same reason. "Well, they said I have to do this and this and this. So I have to do this and this and this!" And then gobs of water changes and chemicals and constantly fluctuating PH levels get some of these guys (and gals!) so stressed-out that they don't end-up worse. Because, from my experience, the number one factor in aquatic illness is STRESS.
These are my thoughts. Just had to throw them out there.

I know that most of us older folks will do our research (from a variety of sources) and reach our own conclusions regarding care, but I worry about the younger set. I know, when I was younger, I would tend to find (what I thought) was a relatively great resource on a subject and look no further. Ya know?
So I guess that's it. LOL Short of it? RESEARCH, ya younguns! Never ever take one source as THE source. Especially forums like this. It's great for discussion and feedback, but leave here and look it up somewhere else! The library is always great! This is great, but when it comes right down to it, this forum is really only a collection of people's OPINIONS. Not saying none of them are valid, but opinions are rarely ever facts.
*end soapbox speech*