I just had to edit a 40 page paper and I'm not kidding, there were paragraphs where every sentence started with "This."
How do good writers learn to vary their sentence length and wording to flow? I don't feel I have this problem when I'm writing from scratch, but when editing, I found myself struggling to come up with new ways to start sentences.
In On Writing, Stephen King said to write how you speak. When we're talking to somebody you don't say "This ..." every sentence. It tends to flow smoothly.
Your story is your conversation with the reader.
I found that that advice was probably one of the most informative in making my writing style very accessible and free-flowing for the gentle reader.
Probably not the fast and easy answer that you (or me for that matter) want, but I guess it just comes down to editing; again and again.
Reread the paragraphs out loud, and you will often feel where the problem is. Then it becomes a matter of restructuring the sentence. It's easy to just focus on the start of the sentence, but see the whole sentence as one. What are you trying to say? Is there another way of expressing the same thing? Could the information revealed be combined with the sentece before or after?
Give it some time, put the text away for a while. Over time it will get better.
Learn grammar.
The more you know about grammatical constructions in the language you are writing in, the more interesting sentences you will be able to create. Remember to vary your constructs from simple noun verb noun to more complicated leading clause, noun verb noun and other constructs. When tempted to use a pronoun, especially one which will result in a general reference rather than an obvious connection to a noun, think about a way to use an interesting noun instead. Make sure to read over your work looking for the same noun too close together in a paragraph and restructure those sentences for interest. The more you know, the more you can vary your writing.
At the same time, don't overcomplicate. You want your writing to be simple and direct. As you edit your writing make sure that you remove everything except what is essential.
My first tip is one I only recently discovered: use a thesaurus. I don't know what it was; maybe I was afraid to use it, like it's admitting that I don't know everything, or maybe I just didn't realize how helpful they were. But as soon as I conceded and started using a thesaurus whenever I felt that my word choice wasn't flowing properly, I found that my writing became a lot more satisfying to read.
My second tip is less clear-cut, and is probably not for everyone. Read some postmodern authors like Pynchon or Nabokov, masters of really varied writing. If you already have, do it again. See what they do to make their writing stand apart. Try to write like that. It's going to be a very poor simulation, but eventually you'll incorporate the bits that you like into your own style.
