Firstly, and most importantly-- choose talent you know will be there when you need them- every time you need them.
That’s not as easy as it sounds.
Consider that since many voice talent websites do NOT filter the talent- the majority of the voiceover people on those sites are non-professional or semi-pro. This does not imply they are not good people or lacking in talent, but it does suggest that they might not be there when you need them. I have auditioned many prospective voice actors who thought it would be fun to do commercial VO, only to discover that they were unavailable when I needed them. Many apparently just give up and move on- usually without notice- so it's best to consider only professional voice actors who pay the bills with their skills. When you send them a script, most likely they are sitting at the microphone the moment your email hits their inbox. That’s a big advantage if turnaround and consistency is important to you, the buyer of talent.
If you’ve got a folder full of voice actor's websites bookmarked, it’s a good idea to check every so often to see if those people are even still online- before you need them.
Most professional VO artists are capable of a wide variety of believable deliveries. You can save loads of time searching the web endlessly for just the right voice, by identifying and using only those actors with a wide range of voice styles. Given the right direction, a handful of talent will cover most of your needs. It’s important to note here that there is a big difference between voice acting and announcing. Acting ability outweighs voice tone in terms of importance, because human beings listen for and respond to believability- but you should choose talent that possess both. Sure, a commanding male “hardseller” works perfectly for many things- just make sure your on-call talent are capable of much more. This also means that should your top guy or gal be on vacation the day you need them, your second to top can likely come close to what you’re looking for, thanks to his or her flexible tone and acting skill.
Turnaround for voiceover is often measured in hours, not days or weeks. Choose voice actors who are prepared and able to deliver on short notice, and you’ll never miss a deadline. Look for voice artists who will let you direct the session live via phone patch, Skype or Source Connect, because this can save hours or days in back-and-forth auditions. Not to mention that it’s often not until the actual session that you or the talent notice small discrepancies in the script, such as typos, names that need pronunciation, copy that is too long to fit in the time frame, etc.
Those issues are quickly resolved during a live session- and you won’t have to pay the talent for a second session to read the corrected script.