Add different values to one textbox

Question

I'm trying to add multiple values and total them in one textbox from different button clicks. For example I have textBox1 and 5 labels + 5 buttons. I want to be able to click button one and it adds the value from label1 to textBox1 then if I click button2 it will add the value from label2 to textBox1.

I can get it to work once but when I try to add more totals to textbox1 from different buttons it clears the total? what the best way of doing this function?