I use a system to track my slots wagering, but I wondered what every one else uses:
If I have to wager £100 on slots, I take my starting balance and subtract £100 (even if this results in a negative). Then I just add the values of any wins as they come along. When my actual balance equals my running total then I have fulfilled the WR.
It means I don't have to do anything if I lose. Also I find I'm never sure if I've remembered to count every spin, so quite often do 1 0r 2 for luck. With this system if the running total and the balance are exactly equal then I'm confident I've done the right number.
It doesn't work so well for Blackjack or Roulette, because you tend to get lots of the same wins.