Are there resources for learning how to write a scientific paper?
Some resources on writing mathematics, which I assume will be useful in mathematical physics, are:

Donald E. Knuth, Tracy L. Larrabee, and Paul M. Roberts, Mathematical Writing, (Washington, D.C.: Mathematical Association of America, 1989), ii+115pp. ISBN 0-88385-063-X. [You can download this (minus illustrations) from Knuth's website: http://www-cs-faculty.stanford.edu/~uno/klr.html ]

P.R. Halmos, "How to write mathematics". Enseignement Math. (2) 16 1970 123–152. [There are copies of this floating around online; I'm not linking because I'm not sure of their copyright status.]

However, in my own experience, guides like these are useful because they point out (1) a lot of things to avoid, and (2) a relatively small number of useful tips and tricks. Developing a good writing style is best accomplished by reading as much good writing as you can (including both scientific and other scholarly writing), writing as best you can, and seeking as much good feedback on your work as you can.

See also Jean-Pierre Serre's lecture "How to write mathematics badly", which is informative, funny, and (since his examples are taken from experience) tragic. Look out for the instance he highlights of "without" being abbreviated to "with."