Because the reason for using a diffuser is to distribute warm air evenly, aiming it at one spot for too long defeats the purpose. When using a diffuser, move it around your head in a circular motion. Some diffusers have "fingers" to help separate the curls. If yours doesn't, use your free hand to finger comb your hair while drying it.
Lower the Temperature
Keep your temperature setting on low or warm while using a diffuser, especially if you have curly hair. This helps to preserve the curl pattern. High heat tends to straighten and flatten your curls and also dries out your hair.
Start at the Bottom
To add volume to your hair, bend over, let your hair hang and start using your dryer at the nape of the neck. Move the diffuser around in the usual circular motion, moving slowly up until you reach the front of your head.
Use Heat Protectant
Add a small amount of a hair-protectant spray or cream to your hair before using your diffuser. Even though you will be using your dryer at a lower setting than usual, you will be drying your hair for a longer period of time. The heat protectant will help retain moisture and prevent frizz. To avoid weighing down your hair, don't use too much product.
Take Your Time
Drying your hair at a lower heat setting will take longer, but in the long run, it's healthier for your hair.