How can I stop my eggs from cooking when I make lemon/lime curd?
The general things to keep in mind are:


Don't mix a ton of the hot mixture into the eggs at once. Add a little at a time. 
Don't use too high heat. It's better to be slow about this than to have scrambled eggs.
Don't overheat before you temper the eggs. Go only as far as the recipe says to. If it's too hot, you might be fine if you temper carefully, but then when you mix it all back together, push it over the edge and cook the egg.
Don't let the mixture in the double boiler get too hot while you're tempering the eggs. If you've got it running too hot, and you take out a lot of liquid, what's left can get hot fast, and then when you pour it back in, you might cook a bit of the egg before you can whisk and even out the temperature.


So in summary: err on the side of too slow and too cool. The worst that can happen is that you have to stir for a long time while it thickens.