I've been taking Tarceva for for nearly five months, and have come up with some things to deal with the side effects I've had.
For the rash: I take a daily low-dosage antibiotic (in my case, doxycycline hyclate), and no longer need to use steroid creams on my face.
For the diarrhea: I've greatly increased my intake of dietary fiber (in my case, raisin bran, bananas, apples, and whole-grain breads) and cheese. Be sure to check with your doctor -- bananas interact with some diuretics, and cheese interacts with some heart medications. Behaviorally, I've learned to take frequent bathroom breaks whether or not I feel any need. I've also learned to pack my briefcase with a spare roll of toilet paper, a jar of wet-wipe pads (witch-hazel for me), and two changes of underwear.
For the gas: I've tried to develop a sense of humor, and my family has learned to either just carry on if I'm embarrassed, or lovingly tease me if they think I'm up for it. For work, or other long periods in public, I take activated charcoal. (But check with your doctor, as it can interfere with absorption of some kinds of medication.)