Doing anything that is clearly intended to maximize the number of losses of your ML team. Here's a partial list:
Playing a player at a position not checked by the Show Recs (Projected) option. Worlds should be able to list specific exceptions, such as min requirements for C (game will set reasonable Cs as DH-only), weak armed RFs, and Mike's "C in RF" system. Playing a C or 1B at SS or CF is, IMO, tanking. It's an active effort to lose.
Every player should be allowed 4 MinL seasons to develop, plus the 20 days to delay the arb clock. After that, the best players belong on the ML team. "Best" is arbitrary. Up to the world to determine how to decide that. Popular vote is probably best.
Having any player on the ML team that clearly does not belong there. Stuff like SPs with 15/15 LHB/RHB ratings. If there is a more reasonable P in the MinL, or on the FA market, "I don't have the money" or "I don't want to spend the money" is just defending tanking. Poor planing your way to tanking is still tanking. Players can be waived or traded to free money, or money can be moved from Prospect to Salary if that's what it takes to field a team of reasonable ML players.
More than one 0(0) P on the ML roster. Again, poor planning can't be used as a reason to justify tanking.
While I'm on a roll, here are a few things that I don't think are tanking:
Low payroll. If you can win 70+ games with a $20M payroll, and you're not doing any of the above, more power to you.
Benching your best players and promoting & playing borderline (at best) prospects. My first few seasons, I played several players I now know never should have left AAA. It took getting them some ML AB and IP to learn that. New GMs should be given a few seasons to learn how predictable development is. I didn't know a 26 year old with a lot of potential is a waste of time back then.
Losing. A Min W requirement is probably the single best rule a world can have. But no matter what, somebody is going to pick #1 in the draft. Not being a great HBD GM is not taking.