Equate every number 0-9 with consonants and sounds. You can make up your own system, or adopt an already developed consonant mnemonic. But for example, 3 would represent an 'm,' 9 a 'p' or 'b,' 4 an 'r,' 7 a 'k,' and 2 an 'n.'
Take a large number, and change each digit into its associated consonants. For example, (3972942) will become (mpknbrn). Try to avoid double consonants if you can. In other words, 99 should be either (pb) or (bp), but not (pp) or (bb).
Add vowels to this string of consonants until they resembles words or phrases. For example, (mpknbrn) can become (mop kan burn) or (map kon born). Because mnemonics aren't graded on spelling, words can be pronounced phonetically. Example: 'kat' instead of 'cat.'
Imagine, in your mind's eye, a picture or scene that goes with these words. For example, (mop kan burn) could become a janitor wielding a flaming mop. Make your image as absurd as possible.
Every time you need to recall that number, remember the absurd and memorable scene that you've associated with it. From that scene - (map kon born) - reverse engineer the phrase back into it's original number - 3972942.