I have an app with Admob ads on it but I find that when I use it the ad almost never refreshes because I don\'t change activities, instead I just update a text view when but
As an alternative you can also have Admob refresh ads for you at time intervals you specify. In Admob go to "Sites and Apps", then click "Manage Settings" for the app you want. Then click on "app settings". There under "Automatic refresh" you can change the refresh rate for ads in your app.
EDIT: with the new admob interface it is under Monetize > My App Name > My Add Unit Name > Refresh Rate (thanks ripegooseberry)