This excerpt taken from the DKS DEF 14A filed Apr 20, 2009.
Can I change or revoke my vote after I vote online or return my proxy card?
Yes. Even after you have submitted your proxy online or via the mail, you may change or revoke your vote at any time before the proxy is exercised by filing with the Corporate Secretary of the Company either a notice of revocation or a duly executed proxy bearing a later date. The powers of the proxy holders will be suspended if you attend the meeting in person and so request, although attendance at the meeting will not by itself revoke a previously granted proxy.