I don't know which is the best, but I recommend subscribing directly to the 3 major credit bureaus (Equifax, Experian and Transunion); or some place like freecreditreport where you can monitor your own credit from all 3 bureaus at one time. Most places that claim to prevent identity theft merely do this for you. You don't need to pay someone to watch your credit when you can do it yourself for much less.
As D&DDisney stated, you can also Freeze your credit. You will need to contact each of the major credit bureaus to do this. You just need to remember to contact them when/if you ever plan to make a major purchase, apply for a loan, or apply for a credit card. The bank/lender will need access to your credit history.
Most people do not know this, but every year you are entitled to a free credit report from each of the credit bureaus. Just go to their sites and you can obtain a copy of your credit report.
Also, invest in a paper shredder. Shred any junk mail, old bank account statements, bills, etc. People who often want to steal your identity will do so by going through your trash to retrieve any type of personal information.
I work for our state's Attorney Generals Office. We have a free pamphlet online that you can print out. You can find that
HERE on page 2. It's titled: Identity Theft Repair and Prevention Kit.