The Seven Step Strategy, from the USPTO:

1. Brainstorm keywords.
2. Use the keywords to find initial relevant Cooperative Patent Classification using the USPTO website's Site Search box . In the Site search box found in the top right hand corner of the home page enter "CPC Scheme [plus keywords(s) describing invention]."
3. Verify the relevancy of CPC classification you found by reviewing the CPC Classification Definition linked to it (if there is one).
4. Retrieve U.S. patent documents with the CPC classification you selected in the PatFT (Patents Full-Text and Image) database. Review and narrow down the most relevant patent publications by initially focusing on the front page information of abstract and representative drawings.
5. Using this selected set of most relevant patent publications, review each one in-depth for similarity to your own invention.
6. Retrieve U.S. published patent applications with the CPC classification you selected in Step 3 in the AppFT (Applications Full-Text and Image) database.
7. Broaden your search to find additional U.S. patent publications using keyword searching in PatFT or AppFT databases, classification searching of non-U.S. patents on the European Patent Office's Worldwide Espacenet patent database  and searching non-patent literature disclosures of inventions using the free electronic and print resources of your nearest Patent and Trademark Resource Center.

If you intend to make, use, or sell an invention of yours, you should find out whether someone else has created something like it already. You have a number of options when it comes to searching for patents. Below are some patent searching sites.