The problem with lying is that it devalues communication itself, decreasing the signal to noise ratio. If what people say has no resemblance to reality, or worse, a believable but false resemblance to reality, then what they say should be disregarded. This takes the once powerful tool of speech, and makes it worthless: everyone might as well be saying "blah blah blah".
Or, if you're lazy, consider this:
"If you don't lie, then you don't have to remember what you said."