When making Yorkshire puddings, I use a recipe that has been passed down from my Grandfather, who was apparently quite the budding chef. My Mum taught me to remember 5-3-1 for the proportions of milk to flour to eggs, as can be seen below.
Whisk it up 'til it's good and smooth and then rest the mixture for as long as you can.
When cooking the puddings you are aiming to get a raised rim, slightly crisp and flat centre. Place some fat in the wells in the pudding tray you are using, I always use some of the fat from the joint that I'm roasting, and get it really hot, almost smoking hot. If you can't do this in the oven put it on the hob.
Divide the mixture into the wells on the pudding tray, and cook at the top of a hot oven. I was always taught not to open the door to check on them during the first ten minutes, presumably as the influx of cold air might deflate them.