There are three "methods" of patent searching:
Classification searching: The USPTO classifies patents into different classes and subclasses ? an index can be found below. This was the main method used before electronic databases became available, and is still considered to be the most thorough. Not that I would rely on it solely, but for most inventions its good to use as a method to help insure you are not overlooking anything.
http://www.uspto.gov/go/classification/ Keyword Searching: You can search patents using keywords. But don't rely on using a simple keyword like "golf" to get any real useful results
Reference Searching: Most patents reference similar prior art, and may be referenced by other patents as well.
And "boolean searching" allows you to combine all three methods into a single search string, if you choose to do so.
All three methods of searching can be utilized while conducting a patent search ? for example keyword searching can be used to help find similar patents which can be used to help locate the most pertinent patent subclasses, for example, then some directed boolean searching to look for relevant patents that might not be contained within the most relevant subclasses, and then afterwards reference searching can be used to ?back check? once you think your search is complete. And all searches should not necessarily be conducted the in the same manner ? note that one patent search might be reasonably complete after searching through just one or two single patent subclasses, while another invention might not be accurately contained within any single group of 50+ patent subclasses. Usually a search falls in between these two extremes.
Sites?
www.freepatentsonline.com might be a good blend of "free" and functional. Google patents can be useful for doing some prelim searching or to conduct keyword searches of pre-1970 patents, but is not at a point where it should be relied upon for thorough searching.
www.delphion.com is the best site but is not free. Advice? Familiarize yourself with the boolean operators at whichever site you use, and realize that "patent language" often differs from "layman language."
A professional searcher may have tools at his/her disposal to increase efficiencies, and should also be experienced, which can further increase search efficiency. A good professional searcher should also be more likely to locate relevant prior art than a layman. But, if you have an internet connection, you usually will at least have access to just about all of the same databases any professional searcher ? or even a USPTO examiner ? has access to, and you should at least be fully capable of conducting a better patent search than at least some professional firms will conduct for you. If you choose to do it yourself, at least take it as seriously as you should and spend as much time as is necessary.
As far as the existing patent is concerned, your changes to it must be considered novel and "non obvious" to make your invention patentable. But your patent protection can only really extend to the differences between the existing invention and your own. If common aspects of the two inventions are claimed by the existing patent, you could still be infringing on the patent even if you could technically "patent your invention." As far as determining infringement is concerned, that depends on an "in force" patent's claims.