If you worked for just one employer and that employer took out too much FICA, you should contact your employer's HR or payroll department and ask for a refund.
If too much was taken out because you worked for multiple employers, there is a line on Form 1040 (line 65 of the 2008 version) to claim a credit for the excess.
If too much was taken out and your employer can't/won't refund it to you, you can submit a claim on Form 843:
http://www.irs.gov/pub/irs-pdf/f843.pdf
Please note that you must have a legitimate reason for claiming a refund. Paying FICA taxes is not optional, even if you do not want to participate in Social Security.