score:1
This answer refers not only to the couchsurfing.org site, but to the plethora of other sites that provide the same function. Among these are gumtree.com and london.craigslist.co.uk and jewgether.org Other sites will undoubtedly join the social phemomenon first uncovered by couchsurfing.org.
There is no need for a grand opus in such matters, but it is important to identify yourself and provide at least some background information about you and the purpose of your journey. And you need to be specific about your contemplated over night stays...
You can use this as a template...
Hello ==appropriate salutation==,
I am contacting you because you are identified on ==web site== as a couch-surfing host in ==name of city==. I expect to be in ==name of city== on ==arrival date== and remain for ==number of days==, hence this email.
Briefly, I am ==your nationality=, ==age==, ==gender==, ==occupation== and visiting ==name of city== because ==why you are there== and will need a place to stay.
Would you please let me know if these dates are available via return email. Specifically, I would arrive at your location at ==time and date==, and these are my inclusive dates: ==list of overnights==.
Kind regards, ==Your full name==
Naturally, the ASL part is optional, but recommended in order to avoid any awkward situations upon arrival. Overall, keep it brief, professional and to the point. Be sure that the host understands all of your contemplated over night stays.
When you get a reply, you can then begin negotiating rates and other matters if needed.
NOTE: I will check out fixing the tag so that it refers to the broader social phenomenon of hospitality rather than a proprietary site.