In 1999 when I was about 12, I memorized 120 digits of pi with little issue. I had some extra motivation: once I got to 20-30 digits, a classmate bet me I couldn't get past 100, and he'd give me $1 for every digit I memorized past 100. Never saw my $20 :(. Pretty sure it was a well-executed nerd-snipe.

Having not practiced at all since about 2001 or so, I can still get to about 30 pretty easily. I didn't use any techniques specifically, I just kept practicing as far as I remember.