Choosing the right focal length is probably the first criteria. Longer focal length macro lenses generally give you more distance-to-subject working space for the same subject framing. At the Canon EF-S 60mm f/2.8 Macro lens' 3.5" or so (9cm) 1:1 working distance, it is hard to keep from scaring insects out of the territory. The Canon EF 100mm f/2.8 USM Macro Lens allows about 6" (15cm) and the Canon EF 180mm f/3.5 L USM lens allows about 9.5" (24cm) of working distance at 1:1 magnification. More working distance is usually better for my typical macro work.