The classic, put ice in the bong. Ice in the water doesn't seem to do nearly as much as ice in the tube, so either drain as much water as you can, and fill it all full of ice, or try my next suggestion in conjunction. This is also why some bongs have pinched-in ice catchers.
I put decorative glass beads into my bongs to act as a diffuser... to break up all the bubbles so that there are fewer big pulses of smoke, and things are smoother. This also helps for icing the bong, as there isn't a large volume of ice in the water in the 'bulb' in the bottom. Of course, use caution if you do this to a glass bong... it can help to hold it nearly horizontal and gently slide them down. I had a large glass bong that was full of these beads, and the downtube didn't break, which had also been a concern of mine
You may be coughing for NO good reason. I call it a psychological cough... I will see the bowl burning, and think "oh crap, I'm going to cough" and I most assuredly would at the first taste of smoke. Terribly wasteful. So, I started closing my eyes, or not looking at the bowl as I smoked... It worked! Occasionally this seems to come back, but I just do the above and am fine.
HOT HOT HOT water in the bong. Once again, use caution if you are using a glass bong, and never 'shock' glass with sudden, rapid temperature change. My friend made this suggestion, and I'm pleased with the results. It appears that the increase of humidity and heat in the bong masks harsh smoke taste, or even the sensation of smoke... This has become my new favorite trick. Sadly, the heat dissipates relatively quickly, so you have to dump and refill with hot water every 30 minutes or so.
Finally, you can put a dab of Vick's vapor rub down into the bong tube. Nice for medicating while sick.
Hope these help someone! Add your own, too