There is another strategy - just publish your invention for all to see rendering it un-patentable.
IBM of all people used to do this for ideas they did not think were worth patenting. By publishing them they stopped anyone else from getting a patent on those ideas. IBM published a magazine for 40 years that is a treasure trove of prior art:
http://en.wikipedia.org/wiki/IBM_Technical_Disclosure_Bullet...
Now anyone can do defensive publication on this site
Unfortunately, that (rather noble) act is untenable in a world where everyone's suing everyone in the big boy department. If Apple and Microsoft weren't trying to kill Android with lawsuits, I could see a gentleman's agreement between the lot of them making this a fairly effective strategy. With the current state of affairs, everyone needs all the linked list patents they can get.
I see it less as being noble and more of a practical strategy for a small business. You can't afford to apply for a patent on everything you think up so why not publish it so at least you will not be locked out of using your own idea by someone who comes along later and does apply for a patent.
IBM of all people used to do this for ideas they did not think were worth patenting. By publishing them they stopped anyone else from getting a patent on those ideas. IBM published a magazine for 40 years that is a treasure trove of prior art: http://en.wikipedia.org/wiki/IBM_Technical_Disclosure_Bullet...
Now anyone can do defensive publication on this site
http://priorartdatabase.com/
http://en.wikipedia.org/wiki/Defensive_publication
I used this strategy with some GPS cell phone ideas I had in 2003 and I "published" them myself
http://web.archive.org/web/20031211124028/http://www.gadgete...