The best methods for increasing the strength of memory are:
- better memory representation (e.g. with mnemonic techniques)
- repetition based on active recall (esp. spaced repetition)
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)