I have a complex relationship with my hydration vest. On one hand, it’s a comfortable efficient way to carry water, food and supplies on long unsupported runs. On the other hand, I think it makes me look like a dork.
More often than not, it’s unnecessary. Most races have frequent water stops with snacks. In my opinion many runners, especially new ones, tend to overhydrate anyway. This is not to say don’t drink while you run, just You are tougher than you think.
That said, when you need one it’s best to know what you are doing. First and foremost, burp your bag. Fill the water bag with water, turn it over so the nozzle is at the top, and suck out the excess air through the hose. Nothing screams “I’ve never used one these before!” like a sloshing hydration vest. Also it will annoy the shit out of you within 20 seconds.
Make sure all the straps are comfortable and untwisted. Twisted straps are a one way ticket to destination chafed as hell, which in turn will make your post run shower a grim torture session.
Finally, be sure to get the brightest neon colored pack you can find. Nothing says “I’m a modern day Pheidippides!” like a day-glo color scheme.