7 Essential parts of an advocacy action letter by My Help Book 2014 (follow-up to 5 Self-Advocacy Tips).  

1. Write to the right person: Make sure you are writing to someone with the power to resolve your problem. Be polite but assertive when asking for names and titles in order to find out who you should be writing to.
2. Describe your problem: Give the background of the situation and how you have been affected. Be descriptive and specific about impacts on you and/or your family or community. But no slang, no sarcasm, no abuse, no personal attacks on the person you are writing to. If you were delayed in writing to get your problem fixed, give the reason (e.g. health problems, time stress from being a family caregiver, or other life events).
3. Say why the problem needs to be fixed: State relevant rules, regulations, and laws. If you need help, find someone via Povnet's "Find an Advocate" or 17 super sources of legal information in BC, or the legal section in My Help Book. 
4. Request a specific solution that is fair, reasonable and possible. Ask yourself whether it is both fair to you and something that is likely to be granted. Also keep in mind how quickly you'd like to resolve the problem; a more difficult solution will take longer than something that is easily granted.
5. Give a deadline: Give a specific date for their response back.  Let them know if you do not hear back from them by that date, this will indicate to you that your proposed solution has been denied and that you will then have to take subsequent action. Important: Be sure to state how you want to be contacted -- by phone, email or letter.
6. Next actions: First emphasize you'd prefer a resolution; that you would rather not take action as you have better ways to spend your time. However, if there's no resolution, you will reluctantly take action such as 'going public' or initiating legal action. Be specific: if going public, let them know you will inform your network of allies, you will use social and traditional media, and that you will seek help from advocacy groups or consumer groups (do research and name them). However, do not share your letter (except for proofreading by a trusted helper) at this point or there will be no incentive for them to fix your problem. Wait for a response back or for the deadline to pass before proceeding with your action plan. 
7. End with a message of good will: State that you look forward to their help and cooperation. 

Other tips: Wait before sending your letter: give yourself an extra day as you may think of something else that needs to be included or changed. Proofread: show your letter to a trusted helper for proofreading if necessary. If you don't have a proofreader, print out your letter and read it to yourself out loud. Read it using a ruler one line at a time to help find typos.

More information:
5 Self-Advocacy Tips
17 Super Sources of Legal Information in BC
Example of income advocacy in extreme situations of need from OCAP
ACORN Canada - for information on issues affecting low income people or families 
Povnet's "Find an Advocate" map.

Disclaimer: These self-advocacy tips are meant for people in serious situations where their rights are being violated and who can’t find an advocate or lawyer. These tips don’t work if you are using them to be a bully, a petty tyrant, an abusive consumer, or to just to be a jerk. If you make a big issue out of a petty concern, to be vindictive, or because you are a prima donna, you will offend and alienate people around you.

17 Super Sources of Legal Information in BC - from My Help Book page 22. Click on the photo for links (or see below). Learn more about My Help Book here.