What you are looking for is called "tail coverage."  It extends the reporting period for your policy.

E&O insurance is a claims made policy, so you have to have coverage in effect when the error or omission occurred and when the claim is made.  When you have tail coverage, no new work is covered, but you can still report claims on past work and it will be covered.  

Click here for a good explanation.  And, here for a blog I wrote in 2007.

I believe tail coverage is a bit cheaper than just paying your ordinary premium for additional years, but it is only available for short period of time, maybe 2 or 4 years.  Ideally, you would like tail coverage to last as long as the statute of limitations for a claim to be filed.  That varies by state, so you would need legal advice in your state on that issue. However, the statute of limitations may be longer than any available tail coverage being offered on the market.  So, you may never be able to completely avoid all risk. 

Best,
Robert.