We will associate some consonant sounds with each digit.
1: t/d/th etc.
2: n
3: m
4: r
5: l
6: sh/ch/j
7: k/g
8: f/v
9: p/b
0: s/z
You will need to learn this. Note that they come in voiced/unvoiced pairs, so you only need to remember one of the sounds for each digit. Here are some (imperfect) tricks to remember at least one of the consonants for each digit:
1: t has one vertical line
2: n has two vertical lines
3: m has three vertical lines
4: f-o-u-r ends in an 'r'
5: The Roman Numeral for 50 is L
6: 6 looks like a mirrored cursive 'j'
7: Two sevens make k
8: 8 looks like a cursive 'f'
9: 9 rotated 180 degrees looks like 'b'
0: z-e-r-o starts in a z
Now, for any phrase, we have an associated sequence of digits. Note that we should ignore vowels and determine the digits phonetically, not according to the actual consonants in the spellings.
e.g:
'Wikipedia, the free encyclopedia' -> KPD D FR NSKLPD -> 791184207591
'Lorem ipsum dolor sit amet' -> LRM PSM DLR ST MT -> 5439031540101
'Dorothy Mary Crowfoot Hodgkin' -> DRTh MR KRFT JKN -> 141347481672
Hence, to form a mnemonic for a string of digits, you just need to come up with a phrase that has the required consonants. For example, for the first 100 digits of PI (taken from a blogpost by Ethan Brown), you can memorise the following five setences:
My turtle Pancho will, my love, pick up my new mover, Ginger.
My movie monkey plays in a favorite bucket.
Ship my puppy Michael to Sullivan's backrubber.
A really open music video cheers Jenny F. Jones.
Have a baby fish knife so Marvin will marinate the goosechick.
It's true that it's not the most natural, and also requires some effort to memorise, but memorising 5 sentences is a lot easier and more reliable than remembering 100 arbitrary digits and can be done in a few minutes. Try it!
Sentence 1:
Corresponding digits:
Sentence 2:
Corresponding digits:
Sentence 3:
Corresponding digits:
Sentence 4:
Corresponding digits:
Sentence 5:
Corresponding digits:
Complete 100 Digits of PI:
I randomly generated 3 phone numbers to try this:
88718 17668: Fave activity? (To) catch Shiva
73643 21375: Kimchi remained Michael
92322 07999: Panamanian psycopath baby
You can make modifications like deciding not to count prepositions, articles, endings coming from verb conjugations etc. as part of the encoding, in order to make it easier to form phrases. It's also quite common to use Hérigone's Mnemonic System in conjunction with the PAO system i.e. instead of forming a random phrase from the consonants, you group them to form a Person/Action/Object/Person/Action/Object... word sequence. The idea is that, each P-A-O block corresponds to an evocative image of a person (a celebrity, profession, fictional character etc.) doing the action (some verb) to the object (some noun), so you can remember a sequence of images instead of a long phrase.