Hi shanon,
If you are going to stand in the page itself, you can just declare a javascript array variable globally(not inside any function), and keep adding to the button values to the array.
If you are submitting the page each time, then you will have to either keep a static list or session variable somewhere. Or one more way is you can just store the previously selected button values as hidden text boxes in the screen each time. So you can keep adding to that.
Thanx and Regards,
Anish K George