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:

1. 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.
2. 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.
3. 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.
4. 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.
5. 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 and relatively concise (don't pad it out with testimonials of elaborate resumes of everyone in the company, but do 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.

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:

1. 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.
2. 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.
3. 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.
4. 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.
5. 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.