First we must believe that sentence length is something we can control independent of content. We can prove this to ourselves either by taking a passage of very long sentences and dividing them up (that's easy) or by taking a passage of very short sentences and combining them (that's harder). Here's a passage from a student essay about Buddy Bolden, the legendary blues man, in various sentence lengths from short to very long:
Short There are some things historians agree on. Bolden played cornet. No one could play like him. His fellow band members said so. Even Jelly Roll Morton said so. Morton was egocentric. Bolden worked as a barber. He had his own business. By noon, he was working on his second bottle of whiskey. You had to go to him before noon if you wanted a decent hair cut. He gradually went insane. He was committed to a state mental hospital. That was in Louisiana. He was committed in 1907. He died there twenty-four years later. (15 sentences)
Medium There are some things historians agree on. Bolden played cornet like no one before or after him could do, for one thing. His fellow band members said so. Even Jelly Roll Morton said so, and he was egocentric. Bolden worked as a barber and had his own business. By noon, he was working on his second bottle of whiskey, so you had to go to him before noon if you wanted a decent hair cut. He gradually went insane and was committed to the East Louisiana State Hospital. He was committed in 1907 and died there twenty-four years later. (8 sentences)
One sentence (which is how the student wrote it) Historians agree that Bolden played the cornet as no one before or after him was able to do (accounts of everyone from fellow band members to the egocentric Jelly Roll Morton confirm this), that he was self-employed as a barber (one to whom—if a decent haircut was important—you went before noon, by which time he was usually working on his second bottle of whiskey), and that he gradually went insane and was committed in 1907 to East Louisiana State Hospital, where he died twenty-four years later.
Short sentences "feel" lots of different ways: earthy, plain, solid, masculine, childlike, simple-minded, choppy, wise, primitive, honest, blunt. Long sentences "feel" the opposite of all those: sophisticated, intelligent, intellectual, scholarly, clinical, educated, fluid, suave, subtle, deceptive, pretentious. Mid-length sentences feel in the middle. Armed with this knowledge, you can decide how you want to be "felt" and choose a length to produce that feeling.
We can all break long sentences into shorter ones, but how do you combine short sentences to make longer ones? Consider a simple pair of sentences:
I went jogging yesterday. I saw a dead deer.
How many tools does English give us for combining these into one? The best-known is the conjunction:
I went jogging yesterday, and I saw a dead deer.
But conjunctions are just the beginning. Consider these possibilities:
When I went jogging yesterday, I saw a dead deer. (dependent clause)
Jogging yesterday, I saw a dead deer. (participial phrase)
During my run yesterday, I saw a dead deer. (prepositional phrase)
I went jogging yesterday; I saw a dead deer. (semicolon)
I went jogging yesterday, saw a dead deer, and (compound verb)
The dead deer that I saw while jogging (relative clause)
Any of these can be combined with any others:
When I saw a dead deer during my run yesterday (dependent clause and prepositional phrase)
Now it's just a matter of forcing yourself to use the tools you aren't practiced with. Think like a skater in training: Spend some time practicing axels, then some time practicing figure-eights. Work on turning sentences into participial phrases, then into dependent clauses, and so on.
Rawlins, Jack, ed. The Writer's Way. 6th ed. Boston: Houghton Mifflin, 2005.