Summary and Review of Sentence Types:
1. Simple - consists of one independent clause and no subordinate clauses:
You crossed my mind.
2. Compound - consists of two or more independent clauses and no subordinate clauses:
You crossed my mind, but you didn't stay there.
3. Complex - consists of one independent clause and one or more subordinate clauses:
If he was angry, I didn't notice it.
Last but not least, we have the compound-complex sentence.
The Compound-Complex Sentence
Compound-complex sentences consist of two or more independent clauses and one or more subordinate clauses:
I like the look of dalmatians: however, I suspect that they are too high-strung to be good pets.
When the weather is warm, my sister and I like to walk to work; however, when it gets cold, we catch the bus.
(opening sentence of Anne of Green Gables by L.M. Montgomery [who knew her way around a sentence]):
Mrs. Rachel Lynde lived just where the Avonlea main road dipped down into a little hollow, fringed with alders and ladies eardrops and traversed by a brook that had its source away back in the woods of the old Cuthbert place; it was reputed to be an intricate, headlong brook in its earlier course through those woods, with dark secrets of pool and cascade, but by the time it reached Lyndes' Hollow, it was a quiet, well-conducted little steam, for not even a brook could run past Mrs. Rachel Lynde's door without due regard for decency and decorum; it probably was conscious that Mrs. Rachel was sitting at her window, keeping a sharp eye on everything that passed, from brooks and children up, and that if she noticed anything odd or out of place, she would never rest until she had ferreted out the whys and wherefores thereof.
Long compound-complex sentences are sometimes incorrectly labelled as run-on or run-together sentences, but they are not if their clauses are correctly combined. If you combine your clauses correctly, your sentence is correct, no matter how long it is. Of course, you don't want all your sentences to be long (you may think Montgomery's sentence above is way too long - but it is correct), but you don't want them all to be short simple sentences either. It is best to use a judicious mix.
A variety of sentence types will always be more interesting to your reader than the same type and length repeated over and over.