• Some Requests for Proposals (RFPs) have a page limit for certain areas, such as the capability statement, past performance, etc. If you get 2 pages for something, fill up two pages. Try to give enough information to the technical reviewer of your proposal. Sometimes, it's okay to go a bit over the limit, but don't go too crazy. If your max limit per the RFP is 2 pages, and you submit 3, you might not get penalized, but if you submit 10 pages, then that's another issue. Some organizations have regulations that allow reviewers to throw out (dismiss) for being non-responsive, and not adhering to page limits can be considered by some contract administrators as being non-responsive.
• Try to show in your proposal that you can actually do what is required in the statement of work. Show valid examples. For example, if I am needing a pilot, don't tell me that you can drive a car, and since both have steering wheels, you can transfer what you do in the car easily to maneuvering a car. It don't work like that.
• If I need x,y, and z to be done, and you can only do x and y, do tell how z is going to be handled. Do you have a sub in mind for z? Who's the sub? What's their qualifications? What's their past performance? This is kinda important.  
• Grammar and spelling counts. Proof read and double check. Enough said.
• Don't be arrogant. Arrogance is such a turn off. It's fine to say that you are the best in something, but don't shove the point past the reader's throats.
• Follow the instructions. If multiple documents have to be submitted with your proposal, submit them all. Even missing just one is grounds for dismissing the proposal.
• If a form is given to you to submit your price proposal and you can't figure out how to use it, well dang. Looks worse for you when it's an excel sheet with the explicit instructions to "put your price(s) in the highlighted cells." Just saying.

Hope this helps. I know I'm looking forward to receiving and reading better proposals.