Zip keeps blowing open or just won't close properly? Give it some love
Most problems with zips aren't actually a problem with the zip. About 95% of problems that I see are with the runner, and these are actually quite easy to not only fix, but also avoid.
A zip is actually quite a simple thing. Teeth that mesh into each other when pushed together by the runner. But if the runner isn't creating enough pressure, the the teeth will only be pushed half into each other - and the zip splits open, or the zip doesn't close at all - and you sit there pissed off with the runner half way up the zip and your trying to fight your way out of the jacket.
Seeing as prevention is the best cure, test your runners on a regular basis. If you feel the resistance as you close them, all good. But if you notice that the runner pulls through too easily, have a close look at it. The runner is simply two parallel plates with a joining piece at the front. If the plates aren't parallel anymore, squeeze them gently together again using a needle nose plier. But careful - better to squeeze 10 times too gently than once too hard. Every time you adjust the plates, test the runner on the zip, until you feel the zip has a nice resistance. Most YKK brand runners can be realligned 2 or 3 times before they need to be replaced. And replacing them isn't hard as shown in this video from Norrona - How to replace a runner.
Another way of reducing the wear and tear on the runner is not pulling it so quickly. The speed builds creates heat through the friction, which will tire the metal of the runner. Another solution is lubricating the zip. Either with a zip-stick from McNett or candle fat. Don't use Vaseline, it will end up spreading a lot further than you think.
If your not sure whether it's the runner or the zip, twist the runner back as you close the zip - this will create more pressure to push the teeth fully into each other. And visually check the teeth of the zip for wear and tear. A badly adjusted runner will wear the outsides of the teeth a lot quicker - so check the runners regularly!