When I took (full-size) flying lessons, I was traveling, and so I had a lot of instructors. The best ones were naturally the older guys with more experience, and there's one tip that kept coming up. It works just as well for RC flying.

Engine failure is one of the most common causes of crashes, catching us by surprise when we're too low to make it back to the field. So...

Think of the "safe" flying area as a cone shape, with it's point being on the ground. In other words, he higher you fly, the further you can get from the runway, and the lower you fly, the closer you should be to the runway. Always stay within this "cone of safety", so that if your engine quits at any point, you can glide back to the runway.

And, related to this problem, is another tip which can save you a lot of hassle. Follow the engine manufacturer's advice, and break your engine in properly. Many new fliers get impatient once the plane is built, and figure they can break the engine in by flying. This is a bad idea! Take the time to run at least 3 tanks of fuel through your engine, running it at idle, transition, and high speed, as the manufacturer recommends, and you will greatly increase it's reliability.