When I write, I take the kitchen sink approach. The first draft is stuffed with physical description, explanation and hundreds of unnecessary adverbs and adjectives. The editing phase is used to transform this monstrosity into something readable.
Adjectives and Adverbs work insidiously to convert tightly written prose into a flowery morass. To fight back successfully we must know our enemy. An adverb modifies the meaning of a verb, an adjective, another adverb, a phrase or a clause. An adjective modifies the meaning of a noun or a pronoun.
Consider these examples:
The line of trucks moved slowly. The adverb slowly modifies the verb moved. This works well; the reader needs to know how fast the line is moving.
The line of trucks moved extremely slowly. The adverb extremely modifies the adverb slowly. I suppose that stresses the fact that the line is slow but, really, how much detail do you require?
The line of transport trucks moved slowly. The adjective transport modifies the noun trucks. This helps the reader picture the vehicles in his mind.
The line of large, yellow, eighteen wheeled transport trucks moved slowly. The adjectives large, yellow and eighteen wheeled also modify the noun trucks. This sentence, no doubt, is descriptive - but what a mouthful! Move a few of these adjectives to another line or, better yet, leave something to the reader’s imagination.
The usage of adjectives and adverbs depends on the type of material being composed. I write fast-paced, two thousand word episodes. Brevity is the key and I try to average no more than two adjectives/adverbs per sentence. If I use two adverbs and two adjectives in one line, later in the paragraph I will compose a sentence which contains none.
If I were to write a historical romance, I’d feel comfortable increasing the number of adverbs/adjectives. First though, I’d browse through a best-selling novel of the same type, checking to see how many that particular author used.