Dear Ned, You gave an excellent example of how to write a good question already:
1. Use a short and descriptive subject. Avoid general formulations like "I need help", "My Matlab function does not work" and "Plz urgent plz".
2. The body of the question should include the minimal number of needed information, but not less. Explain the data available as input and the wanted result (here: "I want get fast and accurate help"). The best method to specify the input data is to show the Matlab function used to obtain it, or some code to create test data. Bad example: "I have an Excel file...". Then it is impossible to predict, if the user knows how to call XLSREAD already, or not. "I want a nice picture" is too vague to lead to matching answers also.
3. Describe the available toolboxes, Matlab and OS-versions, if they concerns the problem. These details do not waste a lot of space: "Matlab 2009a, WinXP, Toolboxes: Signal-Processing, Spline, Financial" - just a line, but might be helpful. E.g. the appearence of GUI elements is often affected by the OS, so mention it. BUT: The question of this thread does not concern any hardware and software details, so such information wastes the readers time.
4. Be aware, that you are very familiar with your problem, but the readers are not. So write your message, relax, and read it again: Is it possible for others to understand each sentence? Bad examples: "I've tried it with a lot of different approaches - nothing worked". Neither the first nor the second part can be reconsidered by others. Concentrate on what you are searching for: one single efficient solution. Therefore write: "I have tried this: [your code] and this problem occurred: [full error message]/[description of results and difference to expectations]".
5. If you can insert a descriptive picture, do it. A handmade drawing can save 1000 words. Use the advantage, that MATLAB Answers is not a newsgroup!
6. If you have problems with a specific command, read the HELP and DOC - twice. If the questions concerns the absolute basics, read the "Getting Started" chapters of Matlab's documentations. Read the FAQ also: http://matlab.wikia.com/wiki/FAQ It is even worth to read it before a problem appears: It contains a list of "frequently asked questions", so most likely these questions will appear in your life also. If the answers are: "read the help", "read the doc" or "read the FAQ" think about deleting your question.
7. Although it is valuable to follow such advices, it is a very common phenomenon, that a question is solved already, if the inputs, the methods and the outputs are exactly defined. So if your question is incomplete, trust in the further inquiries of the readers and answer them. In the first 12 hours after postin look frequently for the questions of people who try to assist you to improve the question until an exact and efficient answer is possible.
8. Either mark the accepted answer or post the working solution, such that other can profit from the discussion also.