Most likely you are putting to much tension on the zipper by packing your suitcases too full. It's obvious that this will increase wear and tear.
Teeth zippers should be stronger than coil zippers. YKK #10 is the strongest there is that I know of.

Put the zipper pulls all the way to the end of their range. This is where the bag is the sturdiest because these areas are often reinforced. Moreover, if this is the bottom of the bag, there are usually feet or a rail to protect the bottom. This will also protect the zipper pulls. When you put them smack dab in the center of the zipper and thus in the center of the bag, they will be most exposed. Ergo, increasing wear and tear. You can also use a little duct tape to fix the pulls on the bag so that they don't dangle. Dangling pulls can get broken off because leverage can build. Flat pulls = no leverage = no breaking.

All these things should help. I've never had a zip go out on me and I use these methods (I don't even bother with the duct tape trick) but mostly I don't overpack. If the bag bulges it is close to overpacked. If you have to pull the lid down to compress the contents and be able to even close the zip it is overpacked. On a correctly packed bag you shouldn't have to pull the zip together by hand so that you can close it.

Till