You'll want the most even and flat lighting possible, so pick an overcast day if it's outside, or use soft/diffuse lights if it's inside.
I read something about someone capturing normal maps by bringing a torch and taking pictures of their texture/material from the exact same angle (use a tripod) and lighting it from above, below, to the left, and right, and then somehow using those to make a normal map by using those pictures as colour channels to build up a normal texture. Sorry if that's really vague. It's all I can remember at the moment.
Edit: I found it. The article uses some old tools but I imagine the process is similar.