...nowadays we carry a PA (speakers, subs, monitors, three pairs of cables, a mixer, a rack case with two power amps, a rackmount power strip and a crossover, two guitar amp mics, two vocal mics, stands and cables), a lighting system (two racks on stands with PAR 64 cans), and various electrical cords and strips; the singer has two guitars and stands and an amp and its stand; the drummer and his drum kit, the lead player's guitar and amp and stand, and at least one bass and stand and my amp and ITS stand....
We bought a trailer, and it gets packed the afternoon of the gig. All the stuff (other than individual instruments and amps) lives in our rehearsal space, so there's no scrounging.
I carry a bass with a tuner in the case, a Fender bag with cords, a 25' electric extension (amazing how many gigs it gets used on), an amp stand, the amp, a bass stand, sometimes a two-step ladder when the stage is very high (I'm an old guy) and a furniture-mover type thing I got from Carvin that's about 2' x 1-1/2' that I roll everything back and forth on.
How do we remember? Years of experience.
You know what they say about experience? It's the good judgment you develop from bad judgment.
Luckily, with this band, the biggest thing we've ever left behind (knock on wood) was the lead player's stand, and someone was able to bring it before the set started.