How to increase the durability of gear-shifters
There are a few ways that user habits or mechanical adjustment can negatively effect durability:

If you downshift the right or upshift the left when the cranks aren't moving, you'll be putting excess force through the internal parts. A little bit of this isn't a big deal, but doing it habitually can break shifters.
Left shifters sometimes fail prematurely due to a combination of too much cable tension and the front derailleur high limit screw being set in enough that it causes the shifter to be held under tension when in the large chainring position. What happens then is that when shifting into the middle ring position, the release lever push is very hard and excess force is put through the shifter.
Some people shift more than others. This has an impact on shifter lifespan.
Shifters need their internal lubrication, and some environments are harder on it than others. Usually this would not explain the kinds of failure you're talking about, but sufficiently rainy/salty environments could prove an exception, especially if the bike is stored outside. Shimano trigger shifters are factory lubricated with a few strategically placed globs of highly washout resistant grease, which can be carefully replenished if things do get dry inside.
Some cyclists ram the shift levers with arbitrary amounts of force and others have shifting technique that's more calibrated to the fact that there are internal parts sliding past each other inside making it all work. The former will wear out shifters faster. Usually the difference will be fairly minor, but again it's conceivable for more extreme cases to prove an exception.

Shifters get more durable the further up in the product line hierarchies they are. Someone riding as much as you describe is generally speaking going to be the kind of cyclist that benefits from getting mid-range or better components, i.e. Deore. That said, all trigger style shifters could suffer and break from the above conditions, and I'd suspect there's some cause other than putting a lot of use through low-end shifters that's causing the failures.
The highest durability compatible indexing shifters you could get are Shimano SL-BS77 bar-ends mounted to something like Paul Thumbie mounts. The ball detent based indexing they use handles force in a very different manner than pawl type indexing, and they are more inured to some of the above problems. That doesn't mean that going that direction necessarily makes sense given the expense, or is the only solution.