This is one of the major topics that is never covered here (or maybe I haven't seen any such threads related to this).
Deciding Units
I have seen many people getting confused about units and how to set it. Well, I've got a simple formulae here - just divide your bankroll by 100 and you'll get your 1 unit cost. Means, if you have $100, you should bet no more than $1 per bet doesn't matter you win or lose. This will give you 100 bets in total with 100 units available to play with.
Never chase your losses
Only asses chase losses! Haha, jk. But still, there's no need for you to go and double your bet every single time just to chase your total loss and get into profits. Trust me, if you eventually turn your gambling habits from being greedy to becoming a smart gambler, you will be in much better profits than just gambling it all away at once and chasing could be really dangerous for your bankroll's health.
Calm mindset
There is no need to feel bad for your loss, even if you lose 1 unit, you still have 99 left, so 2 more wins and you are in profit. Don't go all in on one bet, instead place your bets in parts. Opportunities are available every single day in sports betting, but don't bet like a gambler, bet as a sports investor and apply the rule of "1 bet 1 unit" and I assure you that you'll become a much better gambler one day.
Have an edge over yourself
It's not funny to gamble tbh, some people do it for fun but where's smartness in losing your money? Will you be called smart? No, right? It's better to only use and invest your bankroll in those games over which you have an edge. If you are sure about the game you are betting, then only use your 1 unit on that game, otherwise no need to gamble based on half (or 0) knowledge because we are talking about smart gambling and managing our bankroll. This assures that you have higher chances of winning over losing, and this edge will only help you gaining much more profits in the long run. If you don't know about a game, or yours is a few days ahead, just don't gamble and wait some days and let your expected game arrive.
100th of the total bankroll as a bet might not work for everyone. For instance, I don't have a monthly sum to spend on gambling or something like that, and I don't hold significant sums in my casino wallets. When there's something interesting going on and I want to place a bet, I deposit some money and spend it over a relatively short period of time on a few bets, and that's it. It doesn't lead to uncontrollable spending, and I've never lost too much, so it works fine for me.
And regarding the last point, I think it's smart to gamble for fun because paying for entertainment is normal. If one wants to earn money, one should find a job. Most people who are determined to win from gambling turn out to be losing. And if gambling is entertainment, a person can choose to gamble blindly or to do research, whatever works better to satisfy one's needs. As for not chasing losses and being calm, I agree with that.