Long strings of adjectives can decorate a noun, but there are three choices about how they are punctuated: they can be left alone, given commas, or combined with hyphens.
Hyphenation offers the clearest case of necessity: when a single adjective is formed from multiple words, hyphens connect them into a unit, and prevent the reader from accidentally viewing them separately, each as their own adjective. For instance, the meaning of these phrases changes drastically based on hyphenation:
Don't trust an old car salesman.
Don't trust an old-car salesman.
These phrases can become quite long:
Oh, those latte-with-triple-espresso-shot-drinking yuppies!
To avoid such phrases, you can often reorder a sentence. The subject can be defined using a linking verb (which places the adjectives in the predicate). And a long phrase can be broken up into smaller ones with prepositions.
Don't trust a man who sells old cars.
All yuppies drink lattes with triple shots of espresso.
In some cases, it would be hard for anyone to confuse whether words are linked, and so hyphens aren't necessary. The prime example is an adverb that modifies an adjective: it can't be read as additional adjective in itself.
He cursed his rapidly depreciating portfolio.
But reader expectations -- from context and cliche -- may also render hyphens overkill, in which case your own fussiness must guide your hand.
Her accomplices mocked her novice breaking-and-entering skills.
Her accomplices mocked her novice breaking and entering skills.
Hyphens are helpful, but don't insert them wherever you can. Often, there is no right or wrong answer about combining multiple adjectives together, but the change creates a subtle difference in meaning:
We peered into the deep blue depths.
We peered into the deep-blue depths.
Commas can serve to further separate already distinct adjectives (or adjectival phrases) that describe a single noun. Like the last example from above, their use is a little subtle, and subjective. To use a comma is a choice the writer has, which will alter the meaning and emphasis of the sentence:
The aged military hero climbed onto the float.
The aged, military hero climbed onto the float.
Both sentences name essentially the same person, but the change of one comma shifts the tone, and calls forth a different mental image. In the first case, the subject is a particular kind of military hero -- an aged one. In the second case, he is simultaneously an aged hero and a military hero: the two adjectives have similar weight and exist side by side. By calling out agedness separately, the second case emphasizes it.
Adjectives without commas create layers of description. Like peeling an onion, we read the string of words from the outside, with each layer containing and specifying what comes next, until we finally reach the noun at the center. Consider this example:
Monique loved her knock-off Gucci bag.
We could an insert a comma between "knock-off" and "Gucci" but that would shift the meaning rather strangely. "Gucci bag" functions as a sort of inner phrase: that's what Monique loves, and it happens also to be a knock-off, and so gets that adjective. Here is a subtler case, where both versions could work:
Kids these days waste all their money on overpriced disposable gadgets.
Kids these days waste all their money on overpriced, disposable gadgets.
In the first case, we are specifying a kind of disposable gadget -- the overpriced kind -- and perhaps hinting that some other kind of disposable gadget would be less objectionable. In the second case, we are describing gadgets in two ways; and it is that combination that matters. We may also be suggesting that "overpriced" and "disposable" are the same thing, or go together -- i.e. that disposable gadgets are inherently overpriced, and vice-versa.
A final wrinkle is the expected order of adjectives. You may have been told that this is flexible in English, but that is not quite right: there are categories of adjective that must arrive in a particular order. For instance, there is nothing logically wrong with this sentence:
I'm buying red big new six balloons.
But it grates terribly, and could easily be confusing, because it breaks our expectations. It must be reordered (regardless of whether any commas are used):
I'm buying six new big red balloons.
This is generally easy for native speakers, but the wrinkle arises when some of these adjectives should be linked together somehow. A final example illustrating this, and the variations with commas and hyphens:
You can cook eggs on the red-hot, malfunctioning radiator. (broken and really warm)
You can cook eggs on the red-hot malfunctioning radiator. (a lot of the radiators are broken, but the red-hot one does eggs)
You can cook eggs on the malfunctioning red-hot radiator. (a lot of the radiators are red-hot, but only the malfunctioning one does eggs)
You can cook eggs on the malfunctioning, hot, red radiator. (among all the red radiators, narrow it down to the hots ones, and then the malfunctioning one)
You can cook eggs on the hot red, malfunctioning radiator. (go for the bright pink one that's broken)