Proposal writing is an important part of being a scientist. If you need money or telescope time, you have to make your case that you will do good things with the money or time. It doesn't matter if you have come up with the most brilliant idea of all time, if you can't explain why it's important, you won't get the resources you need to do it.
Most coherent, developed, tested materials
Other important CoolWiki pages
Somewhat less coherent (or less standalone) materials
Actual NITARP proposal instructions (you can see we have converged in recent years and these are all mostly the same now). All NITARP team proposals are available on the NITARP website (well, most of them... some from the earliest years are lost).
Passive voice? -- Dr. Luisa Rebull (2017)
Other sources of interest
Some graduate programs include a class where you both review proposals (sometimes from a real observatory) and write your own. (Many of the links I list here seem to be from such graduate classes.) This is a very useful exercise, as you learn a LOT from reading other people's proposals, both the successful and not successful. It is also really amazing how the process converges -- most people will agree on the 'yes' and 'no' proposals, and then the review committee argues about the 'maybe's.
- https://www.noao.edu/noaoprop/help/sample.pdf - a real example of an NOAO proposal.