I used to worry about this when I was a grad student too. I'm now the lead writer in my scientific collaboration.
You can be comforted that most academic papers are very, very poorly written :)
A couple of things I've learned so far are:
If you want just a collection of "tips" for academic writing in general, I might recommend Style by Joseph Williams. His work applies probably more to essays than scientific papers, but I found his tips for writing very helpful when making my dense papers actually readable (e.g. put all the jargon at the end of the sentence, not the start, make the actor the subject of the sentence etc.). Be advised that most scientific papers are pretty darn unreadable and they certainly don't follow ANY of William's tips, but I include them in mine for that poor grad student trying to introduce themselves to my field.
When I was applying, I got this book by Donald Asher: Graduate Admissions Essays. There were examples in there, but also a lot of practical advice and tips on how to write a successful statement of purpose. I found it really helpful when I was applying and writing.
You can also ask your professors if they have examples of past students' essays to share; some colleges also keep examples on file, either at the departmental level or in some other office. For example, my college's office of study abroad kept old successful Fulbright and other fellowship applications on file, and my graduate school had successful NSF statements on file for us to peruse.