I've been at this for a long time, so there's so many things I've learned that have just become a part of the way I think, and I've forgotten who originally said them. I can't even remember which forum they're on.
I strongly encourage all GMs to take some time going through the In/Off-season Media section and looking at old articles and podcast links. PBSL has a surprisingly rich history, and a lot of us used to be prolific creators back in the day, myself included. Gman in particular has been creating insightful analysis articles from pretty much when he first arrived, taking over from Kucoach, who also used to crunch numbers here and there. Wig, of course, is a legend who basically shaped how this league is played. JNR, BSH, Quail...there's a lot you can learn by reading old articles.
The one (somewhat) recent thing I can remember that immediately impacted my approach was Gman's Guide To UFA (
viewtopic.php?f=7&t=12412&hilit=Guide). For some reason, I'd never noticed that superstars sign first, and didn't structure my offers based around that. That might've been the root cause of some of my Free Agency flops over the years. I immediately changed my approach, and though it didn't always pay off, it did help me with making better use of my cap space. There's a good chance that I don't end up with Amen without reading that article.
I'll also second Gman's comment on Wig's emphasis on timelines. For me, I applied that to contracts and cap space, making sure I never locked myself into a losing and expensive roster, and also making sure that I had the cap to go after key RFAs and UFAs during certain seasons. The first part helped me a lot during the Quintin Kidd years, as I was able to retain my core of Cuomo/O'Neal/Kidd/Cucone without racking up a high tax bill. The second part has helped me be to always be able to go after big Free Agents when I need to, and not pay tax until I have to.