I use a big solid side-address large-diaphragm condenser microphone made by Takstar. It's old now but it was cheap compared to big brand's version of the same thing and it does a good job. I have been using it with a pop filter but I've read recently in Jay Rose's excellent audio post-production book that it's preferable to have it slightly off to the side and do away with the pop filter.
Others use shotgun mics for voiceover, off to the side. I tried this with my Rode NTG-1 and thought it sounded much worse than my Takstar. I expect those getting good VO results with a shotgun are mostly using the Sennheiser 416 which is the industry standard shotgun mic and $$$.
I use the smallest/deadest room I can. I'm currently using a small bedroom with no external walls. When I'm doing voiceover I chuck just about all the soft furnishings and rugs in the apartment into it. Directly behind the mic I have a clothes rack. I have a long cable so the computer itself is in another room. Not as good as a proper booth but it'll do.
When you think you've got enough inflection in your voice, use just a little bit more. i.e. Go a little over the top, but stop short of Steve Irwin. For nature documentary stuff it can help to imagine you're narrating to kids. Lots of amateur voiceovers sound flat (including mine). Starting sentences high can help.
Add a bit of compression to the voiceover track. I have used something like 3:1 or 4:1 with a -18dB threshold.
Finally, don't explain everything that is on screen. I have been guilty of that. If you watch a BBC Attenborough show you'll see there are loads of pauses where the picture does the talking.