How to improve scientific writing skills? What do you do to write better proposals, grants, and papers?

Do you think reading books about how to write scientific content is a good way to improve it?
The way to learn to write is, simply, to write. 

But then get feedback on your writing and re-write in light of the feedback. 

The Software Patterns community has a process called Writer's Workshops that are quite detailed. When you submit a paper to a patterns conference you are assigned a Shepherd who is an experienced pattern writer, usually with knowledge of your field. The shepherd works with you (the "sheep") to improve the paper over three or four iterations of feedback-rewrite. 

After shepherding your paper may be accepted to the conference, though not for presentation in the traditional sense. The conference consists of a set of writer's workshops in which a few (8-10) authors each have their papers discussed by the other participants while they listen and take notes. The author has a very small part in the workshop other than to think about what others suggest about how the paper can be improved. 

After the workshop the author can ask questions, but never gets to "defend" the work. The idea is that if others misunderstand you then it is your job, not theirs, to fix it. 

The paper is then revised one more time and it is this version that makes it in to the proceedings. The whole idea is to improve the print version, not present a version prepared without help. 

The patterns community is pretty close knit because of this working together to improve one-another's work. 

This process was brought to the software development community by Richard P Gabriel who is both a geek (Lisp et al.) and a poet. The same process is used by poets, in fact and is quite old. RPG has written a book on the process: Writers' Workshops &amp; the Work of Making Things

If you don't have the patterns community behind you, or if you aren't writing patterns, it is relatively easy to set up a local writers workshop and follow the process. You can do this for any sort of writing as long as you have some people with domain knowledge and some writing experience.