How do I prevent users clicking a button twice?

I have an old ASP.NET web forms application (.NET 2.0) that has a process that takes 30 seconds or so to run when a button is clicked. I need to prevent the button from being clicked twice. This will prevent posting twice, which causes duplicate data in a database.

I tried adding;

OnClientClick="this.disabled='true';"

to try to disable the button in JavaScript. However, though the button becomes disabled, the postback for the button then doesn't work.

Disabling the button to prevent multiple submissions is the ideal for this application. How can I do this?

EDIT

I am unable to use third-party controls.