Dr. Paul is eligible or in some states certified as a write-in candidate in 11 states :
States: AL, DE, IA, NH, NJ, PA, RI, VT, WI, recently Maine and yesterday California was certified.
In the states listed, varying levels of activism are required for write-in votes to count, from running a slate of official presidential electors to simply writing in the candidate. You should check with your Secretary of State or Registrar of Voters as to how votes are counted ie. as "other" or by name. They should also be able to assist you with how to fill out your ballot.
In California each county uses a different ballot format, you may want to check with your Registrar of Voters as to how to properly write-in your vote should you choose to vote for Ron Paul. Also, each polling place is required to have a list of the certified write-in candidates and should be able to assist you if needed.
Some are suggesting that you make a photograph of your ballot to verify your vote. I like the idea someone had of dedicating a post to show our write-in vote for Ron Paul, President.