Greg Ellifritz writes a great article on Pepper Spray — what to buy, how to carry it, how to deploy it.
Many people buy pepper spray, hook it on their keychain, and think they’re good to go. That’s a first step, but you do need to know how to use it — a little training, a little practice, goes a long way.
If nothing else, whatever you choose to buy be willing to buy at least three of them. Take the first one somewhere where the wind is calm and the spray won’t hurt anyone or anything. Now, spray it. See what your product does: stream? cloud/mist? how far does it go? did it blow back into you? If this isn’t sufficient for your situation, buy another product and try again. If it is sufficient, spray it a few more times to get the feel for how it works and how you can deploy it. Once you have one that works for you, discard the first used can and carry the second (tho give it one spray as a function check). Then keep the third as a backup at home so when the second runs out you have an immediate replacement (and then order more).
Do test and replace your spray regularly. Yes that means you’ll eventually deplete and have to replace it, but if the whole reason you bought and carry it is for your personal safety, aren’t you worth it?