I had an inexcusably bad stay last year at the Doubletree in Santa Monica, CA. Massive, noisy renovations, hot water cut off without notice, room electronics missing, deeply incompetent staff, general manager out of town... yeesh.
I learned the GM's name and wrote him a succinct personal letter, indicating exactly what had gone wrong and what ought to have happened. I then went to the Promus website, learned the name of the VP/marketing (he was new to his job), and copied the letter to him with a formal, personal cover note.
Result: cost of entire stay refunded, offer of two free nights at any Doubletree in the US, and a personal, telephoned apology from the regional GM for Southern California. The hotel was terrible, but I can't complain about the company's attempt to put things right.
The keys, I think: be succinct. Sound professional. Indicate that you are an experienced traveler. DON'T recite an obsessive diary of phone and mail contacts; it makes you sound nuts. Strike a tone not of personal injury, but professional courtesy -- your own. (As in, I'm taking time as a a courtesy to tell you what's happening in Santa Monica under the auspices of your brand... because I felt you would be interested to know.) You get results.
PS. I've stayed in that Wyndham in Burlington as well... with my four-year-old son in tow... always been treated well, never attacked with fire extinguishers, and paid the Wyndham Internet rate of +/- $65. a night.