Peg system – it's a very simple rhyming mnemonic that helps you memorize a list of 10–20 items by quickly assigning visual imagery to each number. It's more of a parlor trick than anything, but it's pretty fun.
Here's the list for ten (but you can go much farther than this)
1 - Sun
2 - Shoe
3 - Tree
4 - Gore
5 - Hive
6 - Sticks
7 - Heaven
8 - Gate
9 - Vine
10 - Hen
Then you take the list of things (grocery list, etc) you want to quickly memorize:
1 - Viking
2 - Ink Pot
3 - Matador
...
and quickly create tiny visuals for each one:
1 - A longboat rows around the spherical face of the sun each of vikings dipping their flagons into the solar flares
2 - A old man accidentally knocks over his ink pot spilling it onto his shoes
3 - An Tree ent holds a cape and taunts the uruk-hai into charging at him
You'll find its trivial for somebody to call out a number and to recall the associated item. Over time you'll also find yourself getting extremely quick at it as well.