Fifteen years of observing these marvels has taught me a few things that I would like to share. First, and foremost, if your instrument of choice is a refractor or cassegrain reflector NEVER use a mirror diagonal with dielectric coatings. They have no reflectance beyond visual red and you will loose the majority of the photons from the cluster's red giants. Recently I have even seen RCs for sale with this kind of coating on its mirrors. Avoid these if you want to use NV devices on globs.
The remaining items are in no particular order of importance. I try to operate at a power sufficient to reach the maximum resolution of my objective. This is 13X its diameter in inches. For my 10" that is 130X (f/13). I will do less if I want a larger FOV. I have found it is a good thing not to be in a hurry. Many times clusters sharpen up a lot with concentrated attention. The faintest stars are tiny pinpoints that sometime evade casual inspection. As far as sky conditions go, the brightest globulars are wonderful under just about any sky but the fainter ones really benefit from good transparency. As always, higher in the sky is better. Last of all, I have never found the need to use a filter of any sort.