How do I get a trademark on a logo and slogan when I have no money?

I have an idea for two great logos and slogans that I would sell to two different companies if I could afford to get them secured through a reputable attorney. I have an attorney in mind. Is it a good idea to ask them if they are willing to secure the trademarks for me at no charge in exchange for a percentage of the selling price? Or would such a request be balked at outright?

First of all, how do you even know those companies would want your logo? You may think it's great, they might not.

Secondly you don't want the trademark. From Wikipedia :

A trademark, trade mark, or trade-mark is a distinctive sign or
indicator used by an individual, business organization, or other legal
entity to identify that the products or services to consumers with
which the trademark appears originate from a unique source, and to
distinguish its products or services from those of other entities.

If you had the trademark it would be useless to the company you sold the logo to. You can have an unregistered trademark simply by using it for business and marking it as such with ™. Actually registering a trademark can take over a year in the US (not sure where you're from)

What you probably want is just copyright, which you already have by creating it (of course you'd need to somehow prove that you created it if a dispute came up).

What you're trying to sell is the exclusive right to use the logo and which would give them the right to use your logo and in doing so establish it as their trademark. What you want is to prevent them from being able to use that logo without paying you. As long as you have a way to prove that you created it and there's no contract saying that it belongs to them, you should be safe. The tricky part is proving you created it, since it is a logo for their company. Google should turn up a number of ways to show evidence that you were the one creating the work. One suggestion would be the keep the original Photoshop files (if that's what you use) or some other raw format that you don't give to them. Emailing to yourself could establish a timestamp.

Disclaimer: I am not a lawyer, and this should not be taken as legal advice.

You can start trading with the design and register it later when you have the funds BUT if it infringes on someone else's mark then it is their duty to defend their mark. Also just because you are using it doesn't mean it can be registered

Pros - you have spent no money and you aren't caught sweating minor details.
Cons - somewhere down the track someone with a similar mark can slap a C&D on you.

Resources
http://www.uspto.gov/trademarks/process/TMIN.jsp watch it
http://tess2.uspto.gov/bin/gate.exe?f=searchstr&state=4008:jrfhb0.1.1 search it