Here's the deal. If you can count something—1, 2, 3, 4…—use fewer. If you can't, use less.
- It has fewer calories than their regular beer. (Fewer, because you can count the calories.)
- There is less beer in this glass than this one. (Less, because you can't count "beer.")
- The patient received fewer analgesics today than yesterday. (Fewer, because you can count the number of analgesics.)
- The patient has less pain today than yesterday. (Less, because you can't count "pain.")
As with everything, it seems, there are exceptions, but they're easy ones. Generally speaking, use less when talking about time or money.
- I've got less than 4 hours left to work! (Less, because we're talking about a time period, not about four individual hours.)
- You've got less money than me. Don't even THINK about borrowing anything. (Less, because we're talking about the total amount of money, not the specific number of bills and coins.)
Hope that helps.