Previous proposals have included:
1. Increase the payroll tax rate
2. Raise the taxable wage cap
3. Raise the retirement age
4. Create mandatory investment accounts
http://money.cnn.com/2006/06/19/reti.../lms_proposal/
And then there is this from fact check during the debates:
http://www.ontheissues.org/Economic/...l_Security.htm