I'd been making jewellery for many years before I learned this simple trick, and I've been using it ever since. I came across it on the UK Beaders Forum, which is a great resource and discussion area for jewellery makers.
When you have designed your earring and are ready to make the loop, add a measure bead to the headpin. The one shown is 8mm, which I find is the best size. A 6mm measure bead would also work, but it would give a smaller loop.
Cut the wire where it comes out of the 8mm bead.
Remove the measure bead, and you will always have the same length of wire, making your loops the same size each time.
Aren't the simple things in life so often the best!
For instructions and photos on how to make the loops, see this blog post, and if you'd like to look at some creative blogs, have a look at Handmade Harbour