How can I get a statement of earnings from previous years?

Hello and thank you for your question.

Of course you could ask your employer for copies of the old W-2's. 
Chances are they keep those indefinitely.

If that's not practical, I suggest you get your earnings history from
the Social Security Administration.  If you had only one employer in
the year, that should match exactly; otherwise you'll need to subtract
what you earned from other employers since the Social Security figures
will be the aggregate number per year.

Request for Social Security Statement
http://www.ssa.gov/online/ssa-7004.html

Search terms used:
social security request earnings

Thanks for letting us help
Google Answers Researcher
Richard-ga

If you are good at keeping paper work of if you have an accountant, it
should be with your copies of your income tax returns.