The most common cause for curdling is the wrong temperature. At no point should you heat the custard to over 87 Celsius, and due to heat inertia, and for a generally better texture, you should stop heating earlier. I have found 83 Celsius to be an optimal target temperature for my taste.
If you preheat the cream (for example you are dissolving caramel in it or similar), let it cool down before mixing with the egg yolks. And after the mixing, when you put it back on the stove, wait for it to reach 83 Celsius (or a bit below, if it's heating up quickly), then immediately take off the heat.
You should also start with room temperature yolks (the curdling occurs at lower temperatures if the eggs are heated too quickly), and never reduce the amount of sugar in a recipe (sugar interferes with curdling).
Acids can also curdle a custard, so if you were using a recipe on the verge of too much acid (e.g. for making a lemon tart), the problem could have been that you have not been precise enough in measurement. But if you tried 9 standard custard recipes, the chance that it is curdled by acid is very low.