It's nice to reminisce about those early days of burning urges and fumbled buttons as you design your birdhouse to be a subtle backyard monument to passion. Not many people know that the most common mistake in birdhouse-building is providing a little perch beneath the entry hole. The reason this is a mistake is that birds swoop up from underneath to investigate the entry hole, and if you've put a perch there, they bonk their heads. The last thing you want to see, as you glance proudly out of the window at your new birdhouse, is a mounting pile of unconscious birds lying at the foot of it. Another mistake is putting the entry hole in an inconvenient location, very much like where God put our mouths.
Birds are extremely particular about the height and diameter of the entry hole.
If the hole is too low on the front wall, the babies might fall out. If it's too big, predators might get in and steal eggs or throw their weight around. Nobody likes a bully, so the hole should be just big enough for the shoulders of the species you are trying to attract. However, if you have trouble getting a measuring tape around the shoulders of your favourite species, it's best to get a book by an author who's already done the measuring. Great birdhouse plans are available in specialized books like Building Birdhouses For North American Birds by John Plewes, available at my favourite weekend hangout, Lee Valley Tools.