Summary
Short: sweet

Sentence Length

Readers prefer shorter sentences to longer ones. However, many writers try to prove their intelligence by writing unnecessarily long sentences. The Long and Short of It

Long Short

Liquid precipitation on the Iberian Peninsula is primarily deposited on flat, non-mountainous regions. The rain in Spain falls mainly on the plain.

The efficiency of Web browsers in displaying graphic images is in direct correlation to the disk resources required by the aforementioned graphic images. Web browsers display small graphic files quicker than big graphic files.

Long	Short
Liquid precipitation on the Iberian Peninsula is primarily deposited on flat, non-mountainous regions.	The rain in Spain falls mainly on the plain.
The efficiency of Web browsers in displaying graphic images is in direct correlation to the disk resources required by the aforementioned graphic images.	Web browsers display small graphic files quicker than big graphic files.

Why are long sentences evil?

Long sentences are evil for the following reasons:

No one really wants to read long sentences in technical prose--they are visually intimidating.
Long sentences are harder to understand than short ones.
Long sentences often obscure the writer's intent (though the obfuscation is sometimes intentional).

What causes long sentences?

I've rounded up the usual suspects for killing your readers' time:

Jamming two or three ideas into a single sentence.
Using passive voice.
Being repetitious.
Being repetitious.
Being repetitious.
Using extraneous words.
Using words or phrases that do not pull their weight; long phrases that could be replaced by a single word.
Embedding lists that should be broken into bulleted lists or numbered lists.

When is a sentence too long?

It is difficult to come up with a magic number or formula for determining when a sentence is too long. A 12-word sentence that minces words can be too long, while a crystal clear, beautifully-composed 22-word sentence is sometimes just perfect. In general, though, never go beyond 30 words.

When are long sentences acceptable?

Long sentences are never acceptable in professional technical communication. Nevertheless, an occasional longer sentence breaks helps sentence variety.

What are the cures for lengthy sentences?

The first step towards a cure is identifying the problem. A good editor will happily nix lengthy sentences. However, if an editor is not available, try reading your prose aloud. When you do, the lengthy sentences will often pop out; you'll be out of breath reading them.

The following remedies are helpful:

Remember the formula:
Divide a long sentence into two or more short sentences.
Remember the semicolon, which is a writer's best friend.
Break out embedded lists into separate bulleted lists or numbered lists.

Note: Many beginning writers try to fix long sentences by replacing collections of words with abbreviations and acronyms. In fact, this often makes long sentences even harder to understand. Don't do it.