1) Talent evaluation ~~ outside of Tim Duncan and Kahwi Leonard (15th pick), not a single rotation player on the Spurs was picked before the 20s of the 1st round, or the 2nd round. They draft and / or find smart players with a specific NBA skill (if not multiple ones).
2) Team construction ~~ no team understands better that it's often a waste of time to reach for "upside" in potential draftees and FAs when they donb't have top picks or huge cap space, respectively. They have a structured system of play, and they acquire based on it. That certainly includes international pros.
3) Financial discipline ~~ they don't overpay role guys. If Jeff Green had gotten his current deal in an offer from the Kings or the Blazers or the Bobcats, they'd have said -- good for you. Good luck. Better to lose you than to be handcuffed bu you. Yes, sometimes they trade for overpaid guys who have a couple of years left, but they always get attached talent back in return that fits the plan in #2 above.
4) Coaching ~~ Pop's the best, and he knows how to tell his team why their team and their system is superior. No egos. No spoiled brats. You buy in as a player, or you sit. The result is high efficiency.