The Delphion web site had a great free utility for searching for patents. The results that were returned were often more relevant than the results from the USPTO. However, Delphion now charges $75/month or more, and the best free way to search for patents is at the USPTO. A good way to search is to follow the steps below:
1) List the main components of the invention you are searching for. For example, if the invention is a computer mouse that is squishy, so as to lessen the effects of carpal tunnel syndrome, your components would be 'soft' and 'mouse'.
2) List all the synonyms and variations of those components. Our example would be 'soft, formable, and moldable', and 'mouse and trackball'. Enter the resulting groups of synonyms into the USPTO advanced search form separated by 'or' between the individual synonyms and 'and' between the component groups. Surround the component groups with parentheses. For example, (soft or formable or moldable) and (mouse or trackball).
3) Click on the link of a similar patent in the resultant listing to get the class and subclass numbers that correspond to your idea. Check these numbers in the Manual of Patent Classification. From the patent text and the Manual get ideas for more possible synonyms. For example, "input device". Remember to surround phrases with quotes.
4) Redo your search, incorporating the new synonyms. Also incorporate the class numbers if your results contain many irrelevant listings. For searching entire classes don't forget to use right truncation. For example, (soft or formable or moldable) and (mouse or trackball or "input device") and CCL/345/$.
5) The list resulting from the last search should contain some patents very close to your idea; hopefully not exactly your idea though. From this list navigate to discover other closely related patents by clicking on patents referenced by patents in the list.