You write the bet down instead of trusting your memory
You leave the pub, open Betgistro and twenty seconds later it is on the record: event, date, bookmaker, stake and odds. When the match ends you mark it won, lost or void, and the potential return, the money at risk and the balance are recalculated for you.
It makes no difference whether you play at Bet365, at Winamax or at both: every bookmaker you use fits into the same list, month by month.