Look to see if you can move clauses around in your sentences. You can often change-up your rhythm by moving a clause from the end to the beginning of your sentence, so long as you don't obscure your meaning. Once you have determined your most common sentence patterns, work to incorporate other sentence constructions. Altering your sentence constructions takes more time than simply altering sentence lengths, but the extra effort will beget extra improvement. In summary, writing is hard, hard work.
All short cuts lead to dead ends.
So, write, rewrite, then rewrite what you've rewritten. And have someone else read it. A fresh set of eyes can be invaluable. Find a scrupulous yet compassionate editor who will tell you the truth. Be wary of those who praise you too effusively.