Here's a seemingly innocent sentence: I now have two less reasons for going. Make it two fewer reasons. If you can count the commodity (two reasons), less will be wrong. You have less justification, but fewer reasons.
Exception: When the amount is one, such a sentence should read, "I now have one reason fewer" or "one less reason, but not "one fewer reason." Admittedly, this is a head-scratcher, but that's English for you.
Use less for specific measurements of money, distance, time, or weight: It costs less than a million dollars. We walked less than fifty feet. Less than thirty minutes had passed. It weighs less than five pounds. The book Modern American Usage explains why: "We take a million dollars as a sum of money, not as a number of units; fifty feet as a measure of distance, not as one foot added to forty-nine other feet; thirty minutes as a stretch of time, exactly like half an hour … and the quantitative less is therefore correct in comparisons; fewer would sound absurd."