The only time a founder should forego salary is before anyone else has invested a dime. And in that case, the paperwork should specify that you are earning a salary but it is deferred, ie you are lending that money back to the company.

Once a real investor enters the picture, if you can't pay yourself even a reduced salary (say 60% market rate), you're not raising enough money and/or you have a sucky investor. Add to the 60% salary with some of that back pay you are owed, or convert the back pay into equity at a favorable (to you) valuation.