Firstly you must be able to appreciate their work, in other words you like the photographs that you have seen on their portfolio and website.
Secondly, you must be able to have a good connection with the photographer. To converse well and explain what it is you wish the outcome to be and not second guess. The photographer although skilled is not a mind reader, so make sure you have a good level of communication to convey all of your expectations, and that the photographer can understand your needs and communicate well with you.
Thirdly, if you get on well with the photographer, the photos will definitely be better and your photographer will feel at ease too. Some of the shots can be quite confronting when the photographer gets close and personal, in your personal space so to speak. You need to feel comfortable in front of the camera.
Fourthly, a list of what you would like to happen on the day is also a great idea. These days I am suggesting to my brides, in particular to have on hand an extra person to help round up people that need for their photos to be taken, this will help to alleviate the stress from the bride and groom and make their day even more special.
Lastly but not least, don't ever compromise on price. After all, your photos will last a lifetime when everything else is just a memory. Your photos will be passed on to family members and down to coming generations. So quality should predetermine all else.