Give As You Earn is a a scheme run by the Charities Aid Foundation (CAF), which allows you to make a regular donation from your salary before tax. Most public sector organisations are already signed up, all you need to do is request a form from your pay department. If your employer isn't signed up, you can download the form from the CAF website.
As your donation is deducted from your salary before tax, you pay less than 80p for every £1 donated. For example if you are a basic rate tax payer a £15 donation will cost you £12, and if you are a 40% tax payer a £15 donation will cost you £9.
More information can be found on the CAF website.