My honest opinion and one I have used myself is first and formost..... wait. Weve been a two dog family for a long time, both were beagles and when the senior beagle died we waited a full year before adding another dog. That gave us and the lone beagle time to adjust to the situation, then we started searching for what we wanted. So figure out what breed has the characteristics you enjoy most and be sure those characteristics fit your lifestyle.
Example; dont get a Great Dane if you live in a one bedroom flat in Manhattan or dont get a teacup yorkie if you plan to hunt ducks.
After you have chosen your breed find a reputable breeder and then visit the sire and dam if possible and then...... let the puppy pick you. It happens nearly every time you visit a litter, one pup will stand out and thats because your personalities mesh. That is the dog you want.
Also, dont be afraid to go to the pound, humane society or a rescue shelter and use the same ideology, in those situations the dog will choose you also.