As I mentioned above, three out of four times it's the readers' fault for not speaking up. For every piece of mail you get, anywhere from ten to fifty other readers simply neglected to say anything, good or bad. Just because you don't hear anything doesn't mean that no one's reading. Practice what you preach. The key here is tact.
Most writers welcome constructive criticsm.
Great, positive feedback is much easier to give than negative. WHAT you liked. Its surprising how ineffective threats are for producing feedback. Well, if no one's reading this, I'll just stop posting. The ones that do reply are probably faithful readers who've just been slapped with a big ol' guilt trip.