The Macquarie Concise Dictionary has the following usage notes in its entry for the word "less".

The conservative rule is that fewer is used with count nouns, and less with non-count nouns: fewer chairs but less butter. However, there is corpus evidence that less is now commonly used with count nouns also, as in this legislation will result in less disputes between builders and clients.

The corpus evidence of which they speak is quite real, and my pedantic streak deplores this trend. I was prompted to write this because I overheard a very brief snatch of conversation as I passed some people whilst entering the elevator they were vacating. The subject appeared to be about the seasons and the weather, and the phrase "wear less clothes" caught my ear. It just sounds wrong, dammit. If it's too warm for you, wear fewer clothes, or less clothing.

There are two tangents I could take on this mini-rant. (Going off at a tangent is better than going around in circles, no?) The first tangent, with which I will dispense in the space of this paragraph, is the question of the evolution of language. I'm not opposed in general to the natural shift of language and usage over time, but I'm annoyed when I see a shift that appears to have been caused by a deficit (metaphorically speaking) in public education. Based on my own school experience, I have a low opinion of the public school system's ability to teach language. This is a tangent that I won't explore in any depth right now, however, since it lends itself to some very longwinded ranting.

The other tangent, which I will explore in a little more depth, is simply to explain the important difference between "less" and "fewer" that most people fail to appreciate (thereby eroding that difference through common mis-usage). I'll explore this tangent because not only does it give me the chance to engage in some recreational pedantry, it also allows me to lecture you subtly in some basic principles of computer science and mathematics.

The most important thing to appreciate here is the distinction between countable and non-countable things. This distinction is not always clear, but it usually is. If you consider people, you can always count them. (Even conjoined twins count as two distinct people.) There is always a distinct number of people in any given group. Contrast this with a quantity of water. Water is not a countable thing, even though it is measurable. You could try to confuse this by saying that even water is made of discrete molecules that can be counted, but in practice you don't usually think of water that way: instead, you measure it on a scale like gallons or litres.

Put simply, "fewer" applies to the countable items, and "less" applies to the measurable items. Sometimes a particular thing can be expressed as a countable or a measurable just by changing the form of the word slightly. I've already given an example of this with "clothes" (countable), and clothing (measurable). Actually, "clothing" isn't something that you'd measure on any formal scale, but you always know that removing some clothing results in a decrease, and adding some clothing results in an increase.

Another example is "clouds" (countable) versus "cloud" (measurable). This is an example where you might choose to talk about it one way or the other depending on the circumstances: if the clouds were formed as clearly distinct "islands" in the sky, you'd probably talk about "clouds" and how many there were. If, on the other hand, the clouds were long and wispy with no clear boundaries, you might prefer to talk about "cloud" and how much there was.

Those of you who were paying close attention will have noticed that I've introduced two new parallel concepts into this discussion: "many" and "much". (Re-read the previous paragraph if you missed it.) These follow the "countable versus measurable" rule in exactly the same way that "fewer" and "less" do. You ask "how many coins?", but you ask "how much money?". Coins are countable: I have six coins in my wallet; money is measurable: I have three dollars and ninety cents worth of coins in my wallet.

I wish to make one final (but important) point on the "countable versus measurable" distinction. You might measure something in centimetres or inches ("how long is a piece of string?"), but when you are talking about the units of measure themselves, they are a countable thing. For example, you could ask, "how much string do we have left?", and the answer might be "six feet". Equivalently, you could ask, "how many feet of string do we have left?", and the answer would be "six".

So if you want to use the "correct" word, given a choice of "fewer" and "less", ask yourself is it "too many" (implying "fewer"), or "too much" (implying "less"). For some reason the distinction between "much" and "many" hasn't been as confused as "less" and "fewer". Perhaps this can be attributed to the fact that "less" and "fewer" have the same opposite: "more". There's no distinction between countable and measurable with "more". You can have more apples in your basket, or more applesauce on your pork, whereas you should have fewer apples in your basket and less applesauce on your pork. "Much" and "many", on the other hand, have distinct opposites in "little" and "few".

As I said before, many people are ignorant of the distinctions I've described here, resulting in much improper use. I hope this little bit of information, these few words of advice, have granted you more understanding of the issue. Perhaps now you will make fewer mistakes thanks to your increased awareness, and thus make it less of an issue for pedants such as I.