Do some research and find someone who has awesome work. Styles are unique to photographers and you want one that resonates with your company image. A great portfolio is essential.
Get to know your photographer. Meet with them face to face, have a coffee (or a wine) and just chat; it helps if you get along.
Find a photographer that is full time. You don't want some fool messing up your big event. None of this 'weekend photographer' stuff. You want someone who is passionate and committed, energetic and fun, with the experience and professionalism to make your event perfect. Follow their blog and see what they're up too.
If you've made it this far, you're doing well! Get on the phone and book them before someone else does.