The spur needs to be timed and applied correctly. Let's say you've used your leg aid, but your horse has ignored it. You would need to wake him up to your aid by pressing the spur into his side immediately, or tapping him with it, so that he will respond to the natural aid the next time. If you simply dig in and dig in, or clamp your legs and spur on him, he will become dull to both your leg and spur. If you nag at your horse, you will eventually make him dull-sided.
Spurs vary on severity by how they are designed.
They are worn level, with the seam of your boot, and buckled toward the outside of the foot. Grand Prix dressage riders may ride with more severe looking spurs, such as one with a smooth rowel or a very long neck. These are worn to refine their leg aids, and you will notice that the horses at that level are extremely light and responsive to both leg and hand. Keep in mind how your horse is built, and where your leg rests in relation to his side. If your horse is large and you have a short leg, you may have to turn your toe out just to make contact with the horse.