Looking at this from the other side, as someone who has tended to be on the buying rather than the selling side, here's what I want from a proposal:
- A clear indication of the scope of work, which should reflect what I have asked for (but not just a cut and paste from my words). I want to be sure that you understand my requirements, and also understand the context of my request... i.e. I want to be sure that you appreciate how the project's success or failure will impact on my business.
- The cost that I will pay, clearly described in terms of whether this is an estimate, a firm quote, what the components are (hardware / software / labour / project management / etc etc etc). - And also what criteria would cause a variation from the cost. I don't want to be paying for your team to get trained on the specific technology because I am the first customer that you have tried it on. And don't forget to mention the ongoing costs of licences, support, etc. That is as important to me as the up-front capital cost.
- Some indication of the experience of your team. I want a team that has proven skills, at least across the main players. I will accept some less experienced people as long as I am not paying full rate for them... perhaps. I also want to be sure that you are not going to put people into the project up front, then pull them off when a new project comes along, substituting less experienced people or making me share them with another customer.
- Your technical approach. If I have asked for a particular technology, you should reflect this back to me, or explain why you think you have a better solution. If I have not been specific, I want to know what approach you will take. I need to be sure that it is consistent with my strategy, or if not, how it will impact on me and my team.
- I would also like to know something about your organisation: who I will be dealing with and where any issues can be escalated to if necessary. I need to know why I should use your company rather than your competitors, and how you will help me to sleep at night without worrying whether I made a poor decision.
The proposal should be well structured, perhaps mirroring the 5 points in my answer, and relatively concise. Please don't pad it out with multiple testimonials or elaborate resumes of everyone in the company. Just tell me what I need to know. It should be spell-checked, and grammatically correct. And if you are emailing it to me, don't fill it with elaborate graphics that will clog my mailbox, or use unusual file formats. A simple MS Word document or a PDF file will do nicely!
Good luck with your proposals. And by the way, I have written this answer based on the assumption that you are selling your services to an external customer, but I suggest that the same principles would be equally valid for an internal customer, but with perhaps a different emphasis or weighting applied to the different parts of the document.