Principle 1: Be generous with your spices. Spices not only bring flavour but texture to dishes. Most supermarkets sell spices in misleadingly small containers. You can buy bigger packets from Asian supermarkets, which will encourage you to spoon in the spices with a freer hand. (You can store them in the freezer to stop them going stale.)
Principle 2: Decide how you are going to cook your onion, ginger, and garlic. This triumvirate provides the deep base flavour of most curries, equivalent to onion, carrot and celery in the French tradition. (NB: garlic is not essential. Some Indians eschew it completely on account of its pungency and it is often left out of food served at weddings to avoid offending guests.) Soften them without colouring for a lighter curry (as in the first recipe) or cook them longer and caramelise (as in the second) for something richer and darker.
Principle 3: Decide what is going to give your curry sauce its body. This will normally be one, or a combination, of the following: tomatoes; pureed peppers or chillies; yoghurt or cream; coconut milk; spinach, or finely diced or pureed onion.