One of the best ways to learn tactics is to coach others during matches. It's a different vantage point that forces you to really open your mind to tactics going on in a match where you aren't playing, both tactics that are being used and ones that are not. It's especially helpful when coaching players near your own level, but you can learn a surprising amount even coaching much lower-level players. And if you happen to be coaching a stronger player, well, there's a lot you can learn there if you are striving to reach that level. Here are two ways you can learn by coaching others:
First, when you just watch others play, you aren't forced to really analyze what's going on. Sure, in theory you can, but do you really? Most just watch the rallies. However, if you are coaching one of the players and are going to coach them between games, you suddenly have strong incentive to watch closely and analyze what's really going on. And from this, you can learn from both the player you are coaching and his opponent. You may be surprised at how much is going on out there, both intentionally by smart players, and stuff the players at the table don't see but that you do, now that you are paying attention - and you can learn from this. What you learn by coaching a player not only helps that player, but helps you as well.
Second, it's easy for a player to get stuck in his own little tactical fishbowl, doing the same things over and Over and OVER, never realizing there's a lot more he could be doing. There are literally zillions of tactics out there, and while you aren't going to use most of them, you should be ready to use many, depending on your opponent. You need to be out there seeing what others are doing so you can pick up on some of these tactics. You won't do this by just doing the same tactics you are used to over and over; you have to get out of your fishbowl and see what others are doing.
A corollary to this is that if you are stuck in this tactical fishbowl (often without realizing it), others on the outside can see what you are doing, and a good coach or experienced player might be able to help. So it works both ways - you can learn by coaching, but you can also learn by someone else coaching you - and if the latter, he learns by coaching you! Call it the "Circle of Coaching."