Guide them through how you are going to solve their problem. Once you've reiterated their pain, show them how you are going to relieve them of that pain. This does not mean that you should start spouting off the tools that you are going to use. Rather, you should be outline the results your client will see when working with you. Show them how you are going to meet their goals and expectations. Explain to them what metrics you will use to determine success. Let them know that you have their best interest in mind and that you know what you are doing. Now, you'll notice that there is no mention of price. That's because price doesn't matter. What truly matters to a prospect is the value that you offer and the relationship that you have built.
When you are writing your proposal, attention to detail is very important. You must write everything in clear and concise English. You should never use slang and only use the technical terms that you must use. The person who is reading your proposal may be a lay person and become confused if you use too many technical terms or job related slang words and phrases. Color for your graphs and charts is quite acceptable and as it eye catching can mean the difference between submitting a wining proposal and one that ended up in the "also ran" file.