Practice working on expanding your texture "vocabulary."  A good cartoonist should have a repertoire that includes many kinds of surfaces, from shiny to hairy.  While there is no substitute for observations made from still-life drawing and such, cartoonists need to convey the idea of something more than how it may actually appear in real life (for example the traditional "window" highlight on an apple).  It's perfectly OK to study how other artists have tackled different textures.  You may even find that you like how one cartoonist does "reflective metal," and how another does "tree bark," and so forth.  Just be sure to try to understand why they drew what they did.  Don't copy it directly (this is one of the Cardinal Laws of Cartooning).  One of my art teachers, comic book artist Bill Sienkiewicz, told me to "Copy what they know, not what they do."  Black and white photographs can also help.  In fact, Dover Publications has a book titled
Textures, by Phil Brodatz, that has helped me on several occasions.  Your local book store should be able to order a copy for you, or your local library may have one available (ISBN 0-486-21669-1).  The book has B&W photos of everything from woven aluminum wire to plastic bubbles.  What I've often done is purposely choose to draw something with many different unfamiliar textures just to try out different ideas.  If you are interested in superheroes, DC's classic Silver Age hero, Metamorpho, is a perfect subject for such an experiment. As always, keep your old drawings (if you can't keep the originals, then photocopy or scan them to CD) so that you will have a handy reference "book" of your own.