We can roughly describe forgetting with R=e(-t/S) where R is [memory retention]?, S is relative strength of memory and t is time.
The best methods for increasing the strength of memory are:
Good representation makes memories "easier" to remember.
Each repetition in learning increases the optimum interval before the next repetition is needed (for near-perfect retention, initially repetitions may need to be made within days, but later then can be made after years)