It's an elementary tip, but learning to think of chords as numbers was a game-changer for me, common chord progressions suddenly made sense, I could transcribe to any key, communication with others was much easier.
Numbers was also a game-changer for me. For most gigs I bring my tablet that uses a setlist app that's basically a glorified notepad. It contains hundreds of "charts", all written in numbers. I include the usual key, but with numbers I usually have no trouble transposing.
The problem for me is that I've become too accustomed to numbers. Numbers work great for the vast majority of rock/pop/country/R&B (etc) tunes, but there are exceptions where they don't -- not on the fly.
One example: I was doing some sub gigs with an 11-member band (5 piece horn section). They would sometimes take requests for songs that I didn't know. They would usually play them anyway: because the band was so big, and had a keyboard player, it didn't matter if the bass player was faking it once/twice a night. The guitarist coached me through the unfamiliar territory. He initially asked "would you prefer letters or numbers", and I said "numbers, please!" Because I'm much better with them than letters. That worked fine until the requested tune was "Just You and Me" by Chicago: multiple key changes! Coaching that one was too tricky on the fly (letters or numbers) so I mostly sat that one out (and made sure to learn it afterward).
