Book them early and relax knowing you have one less thing to organize. Most wedding photographers are independent and they can only do one wedding per day so can often be booked up in advance for popular dates.
Of course make sure you like their photos but make sure you like them too. They will be working with you for possibly the whole day. So a friendly face, relaxed under pressure is a must.
Are they qualified and insured? Ok, their best qualification is their work however a qualified and fully insured wedding photographer is a sign they know what they are doing.
Have a look at photos from a complete wedding and make sure they are of the best quality from start to finish. Everyone can have a lucky photo or take a few shots of models on a training day. Ask to see some albums or slideshows of a whole wedding.
Last but not least, price. As with most things you get what you pay for. If a wedding photographer is super cheap they will have to cut back on things like the final quality of the photo, the album presentation, the level of service to you and your family, possibly even cutting corners backing up your photos after the wedding. Price is important and if it means spending a little bit more to be ten times happier that's the way to go!