Incompetence comes in many shapes and sizes, all of them ill- fitting. If your relationship with a fellow worker isn't producing adequate results, examine the causes. Don't let personality clashes or differences in outlook obscure the person's abilities. The better you can recognize a co-worker's skills, the better you'll be able to work together productively. Any time you find yourself remarking on the difficulty a co-worker would have in finding his way out of a paper bag, hold on.
Negative comments reinforce negative behavior.
And your opinion may cause others to prejudge the worker unfairly. Reinforce these points as unmistakably as you can. It's natural to feel put upon when people perform poorly, but it won't help to lash out at them. Whether people are aware of their shortcomings or not, harsh criticism can only make circumstances worse. Keep your comments constructive and related to performance.