Building a martingale strategy: outcomes choice

By taking into account successive results, in particular losses, we can build a strategy of betting on several and successive moves, and overall always winning: this is called a martingale.
The goal is of course to try to increase your (chances of) winning. The ultimate goal is to win every time.
Regarding martingales, we often see (always, it seems to me) that we must always bet on the same event. The most commonly cited example being the classical doubling martingale in roulette: we always bet on red for example, telling ourselves that it is bound to happen eventually: the red will eventually come out. To build a martingale, it seems that the most important thing is to always choose the same result, red here for roulette, and absolutely not change your mind.
This is an error in mathematical reasoning. In a martingale, the important thing is not the outcome of the event, but the stake ! Indeed, if we suddenly bet on black at roulette, it would not be offended. On the other hand, the stake must be sufficient to both reimburse previous losses and at the same time allow a net profit. It is therefore the calculation of this bet which is the crux of a martingale. Of course it is also useful to put this into perspective of the estimated probability of gain/loss.

A martingale stopped because of the end of the season ?

Stopping a martingale means losing, possibly big if it has already lasted a little. We must therefore keep in mind the objective in a martingale: not to stop it halfway !

A typical example: in a martingale like martingale of draws, some people think they have lost their martingale because the season, or the championship, is over and their martingale had not yet been successful.
It doesn't actually matter the "draw" outcome (the interest of which lies mainly in having a fairly constant odds of a little more than 3) and the season, the championship, etc. This martingale must be continued with other bets, as long as we continue to respect the stakes.

If the events are independent, the probabilities of loss multiply (like the probabilities of winning in accumulator bets) and therefore the probability of losing the series continues to decrease, regardless of continuing a "draw" martingale with matches of tennis, hockey or bridge.
What matters is respecting the amount of money that is wagered to maintain the targeted net profit. Si les événements sont indépendants, les probabilités de perte se multiplient (comme les probabilités de gain dans les paris combinés) et donc la probabilité de perte de la série continue de diminuer, peu importe de continuer une martingale "match nul" avec des matchs de tennis, de hockey ou de bridge.
Ce qui compte, c'est de respecter le montant des mises pour maintenir le gain net visé.

The following martingale calculator Calc link calculates the adapted bets and the probabilities of successive losses of a martingale.