WIF Logout - How to?

I create principal like this:

ClaimsIdentityCollection identities = new ClaimsIdentityCollection();
identities.Add(new ClaimsIdentity(claims, "Forms"));           
var principal = new ClaimsPrincipal(identities);

var token = new SessionSecurityToken(principal);
var sam = FederatedAuthentication.SessionAuthenticationModule;
sam.WriteSessionTokenToCookie(token);

I tried signing out like this:

FederatedAuthentication.SessionAuthenticationModule.DeleteSessionTokenCookie(); 
FederatedAuthentication.SessionAuthenticationModule.SignOut();

But user still appears to be signed in. Anything I'm missing?