Best thing, get a timer! I do on my two big tanks (out of 3 running) And its brill, cos you can be totally precise about how long its on a day, and you dont ever forget!!! Timers cost about £8 for 2 (apparently my flatmate has just said).
yup i got mines on timers. main tank has timer for lights and another times set to run a bubble attraction for 15 min every hour or so. other tank has normal day light and moonlight lighting set to run x amount per day. that way i never worry about it.