I will be going to a glasshouse where there will be butterflies as part of a special exhibition / attraction.
What are the best techniques for taking good shots, assuming that using a tripod might be difficult? I will be using a 100mm macro lens.
I would definitely bring a monopod. Where a tripod might be difficult to use given space constraints, time constraints, or regulations of the exhibit, a monopod should give you a considerable amount of stability while still being very mobile.
If you are inside of the glasshouse (green house), you should be free to use flash. I would either use a macro flash, or find a way to move your flash off camera, close to the subject, with a small diffuser. That should help increase lighting, which even in a green house is likely to be dimmer than you would really need. You won't need a large light...but a small diffused flash should be perfect for butterflies.
If you are going to be shooting through glass, you'll want to make sure you shoot at an angle that hides reflections. It can be possible to use a flash through glass, especially if it is off camera, but you run the risk of increasing glare and obscuring your subject with reflections. You'll want a fast aperture and higher ISO if there is limited light.
What brand are you with? The canon 100mm L macro has image stabilization so you might get away with a monopod or even handheld at fast shutter speeds. Personally I always use a tripod for macro with my canon 100mm non L macro lens.
EDIT I used to have that exact setup. I would try using a monopod with fast shutter speeds if you cant use a tripod.