The worst thing you can do is show your opponent that you're tired. Try not to get too involved and put on a pokerface.
I usually end up in the ropes when tired, the ropes elasticity can be used to slip without expanding too much energy (eg. lean backwards).
If he needs to worry about being tired for "minutes" of a round, they are screwed. All I would do is cover up and throw a one two combo and then move and cover up again.
There's no one answer. Running works well. It gets you away from danger, and it takes less energy than fighting. Clinching can work really well too, but if you got a guy who knows what to do in the clinch, or if he's bigger and stronger, he'll turn that around and you'll be wasting a lot of energy wrestling him. Covering up works okay, just don't let the guy work you over too much. Fouling...it works, you could lose a point, and it's a scumbaggy thing to do, but it could get you a break. (the opposite of this is fake being fouled, if done successfully, you get 5 minutes to rest!) Take the Knee. Again, you'll lose a point for this, might be worth it though if you are on the verge of dropping anyway.
What he said^^ just try to relax yourself as best you can and take some deep breathes and control your breathing. afterall breathing is the only thinig thats gonna bring that wind back.
To be honest my conditioning is not that good but during the three minutes I rarely get too tired to function. Staying relaxed and staying at a proper distance at all times while breathing constantly should allow you to keep you going no matter what. If you're in a major brawl and taking punishment to the body that's one thing, but in general even in a relatively fast-paced match mentality is most important. Personally, like I said my conditioning is not that good, but I'm too focused on what's in front of me to think about how tired I am. I'm still trying to throw crisp punches and not get lazy. I feel it from time to time, and by the time I rest I'm panting, but with relatively good conditioning focus should be enough to mediate fatigue.
You've just got to keep going. Try your hardest. Analyze what the other guy is trying to do and defend it and pick off your punches. When running i practise maxing out my oxygen and then recover. Do it over and over again and you can learn how your body responds to stress. I dont know if that helps you but i find it helps you get more feel into how you exert the energy to get it back.
Very good thread The pros through experience know how to pace themselves. They aren't going 100% for 12 rounds, more like 70-80%. Fact is their 70% is more intense than our 120%. Just like elite marathon runners can run 42 km faster than we can run 2 km. Rookies tend to go all out too early, throwing every punch as hard as they can and wonder why they gas. I do a lot of training teaching myself to punch non stop at 75% intensity. It's harder than you think to master because any kind of emotion usually makes you hit out too hard.