1. Know your limit
Have a yearly bankroll limit which you won't cry over losing and if you do lose it wait until next year to start again.
2. Bankroll management (unit betting)
Only wager 0.5%-4% of your bankroll per wager. For example if your yearly bankroll is 10,000$ you should wager 50$-400$ per wager. Gambling to win is a long game, you will lose many times, if you bet a large percent of your bankroll each time that's less plays until you're broke.
3. Get a good price
Don't wager on outcomes that require you to lay down too much. Try and get at least 166%. i.e. if you wager 100$ you should profit 66$ (get 166$ back)
4. Wager on an exchange instead of an online book
Exchanges such as Betfair, Betdaq, Matchbet, Smarkets won't refuse you if you win too much. Another benefit is that you can request odds.
5. Wager on what you know
Hockey and baseball have been my bread and butter for years and I've stuck with those. I love basketball and I think I know it well, however according to my record I don't, so I rarely touch it if only for fun.
6. Don't overpredict
I'm not Nostradamus. I know it pays out more if you wager on really specific occurrences in games but I avoid wagering on things like who will score a touchdown. Same for parlays, I avoid them for the most part. I just wager wins/losses and overs\unders that's it.
7. Make congruent wagers
I would never wager on a hockey team to win by 2 and on the score to be under 5.5. I know it happens 3-1, 4-1, etc. It's just that a team is more likely to win by 2 in a high scoring game.
8. Don't "overMartingale"
Know when to cut your losses, don't overchase. If you lose 50$ and bet 50$ to get that back, lose that, then bet 100$, lose that, and now 200$, lose that, and 400$... Stop the chase at 400$ which would be about 4% of your bankroll if your bankroll is 10,000$.
Those are some of my thoughts regarding wagering. These guidelines have helped me over the last decade. I'd love to read other gamblers thoughts about their personal gambling policies.