Surely not the only ways, but here are some ways to give direction and organize your collection. Feel free to post other ways or how you collect.
Fancy Serial numbers.. If the collector knows what they are exactly. http://www.coolserialnumbers.com
Emergency notes, locals, notes from a particular printer or monetary authority, one from each country, omnibus issues, birthday year, notes designed by a particular artist
Since I'm keeping grandchildren this weekend thought I'd ask the 6-year old his ideas and he thinks this says it best:
Red, the color red, among others. And famous people like Patrice Lumumba, Jomo Kenyatta, and Kwame Nkrumah. Heavily loved "teddy bears" (torn and multiply-taped).
Faraway places with strange sounding names, calling, calling... And, as microbiologist... dirty notes might contain hardy viruses, eu-and prokaryotes, archeobacteria, fungi, insect eggs, depleted uranium from the Gulf War and cocaine from California $100s... the possibilities!
Nice note @Clarenze Cruz . I don’t necessarily collect seven-in-a-row, but felt extra Lucky after receiving this directly from the BEP as a Lucky Money note.