This article will not be about the kind of momentum you get when you go on a win streak. I am not really a believer in that kind of momentum even if happens, because it's not actually momentum. When you go into a new game, match, whatever with the exact same options as your enemy, everything is neutral except what is going on in your head. We're not talking about the head stuff today.
Instead, we're talking about events that happen during a match. The #1 worst thing you can do in a match is give up momentum. In a FPS game like Halo, Quake, or UT when you're ahead, you don't let your opponents get good weapons. Ideally, you don't let them get any weapons. In a fighting game, if you knock your opponent down or put him in the corner you make him struggle before he gets to play again. No matter what situation you're in, no matter what game you're playing, the best way to win is to not give the opponent a chance to play.
I'm dead serious. If unplugging the enemy's controller was a legal move, the best move would be to do it. Take away your opponent's moves and don't let him or her get them back. Of course, unplugging the opponent's controller is not a legal move, but there are plenty of ways in the game that you can deny the opponent options. The exact method varies by game but we're here to talk about the opposite, letting the opponent play.
One thing I really like about LoL is that you can give your opponents an apparent opening and punish them for it. It's very similar to traps or resets in fighting games. What I mean is that you create an opportunity for your opponent to catch a breather or make a play. In high level play, you see the other team immediately attempt something. You can guess based on your stranglehold of the map (wards let you know where the enemy isn't, even if you can't see them) where the enemy team is, and play to punish their breakout attempt.
For instance, if you're really ahead you might see the enemy team group up to take an objective like a dragon or turret, and your team split pushes a bit and turtles while two of your team go to duo Baron. You kind of bait the easy objective while you take a better one. Another example is the death bush. If you know the enemy team wants to set up a death bush and you haven't seen them for a while, you can use that opportunity to take objectives somewhere else.
That is the point, though; all of these are tricks that you're using to get the enemy team to hang themselves. If you're losing momentum, it should be because either the enemy team made a great play/you screwed up (it happens) or because you're trading momentum for something tangible like a resource advantage. We see it quite a bit in StarCraft where a player wins a little skirmish and could possibly push it further, but he decides to back off and expo safely instead. A safe expo is the key to more momentum, so it's good to trade immediate power for.
On the really micro scale, take a situation in LoL where you're winning your lane. If you are quite a bit ahead and are getting good damage in on your enemies, don't let them farm. It seems obvious, but I'm serious. Don't actually let them scale up into a state where they could be a threat. You can't prevent someone from farming under a tower in LoL, but if you have done enough damage you can set up a dive and keep them from playing even more.
Of course, this all stems from fighting games. If you make your opponent block something, do your best to not just let that one blocked jab go. Sometimes you are going for footsies and they block, and you might not be able to get too much from that (depending on game; most "anime fighters" have lots of ways to get momentum on any blocked hit) but if you can confirm anything into a mixup, pressure, any sort of lockdown, do it! Don't just end your string in neutral, have ways of mixing things up so you can stay in control. Force your opponent to make a guess in order to stop your offense.
Part of that means that you need to make your offense not airtight. As I said above, you need to give a tiny bit of space to let your opponent hang himself. If the only option is to block or turtle at a turret, then that's what your opponents will do, but you're losing out on ways to get ahead and take the enemy down.