Amphibians have moist glandular skin. The wrinkles, warts, and folds of the skin are often important details for identification and to include in your sketches. Some species have a hairline crack between the lip and the nose. Also look for a flap of loose skin under the head that makes a fold behind the jawline.
Think of the eye as a solid sphere. You need to wrap the eyelids around this shape. Seeing of the eye as a three dimensional form will help you get the look onto your paper.