You can use a selector that specifies the drawable for the various button states:
Using java Syntax Highlighting
<?xml version="1.0" encoding="utf-8"?>
<selector xmlns:android="http://schemas.android.com/apk/res/android">
<item android:state_pressed="true" android:drawable="@drawable/btn_minus_pressed" />
<item android:state_focused="true" android:state_pressed="false"
android:drawable="@drawable/btn_minus_selected" />
<item android:drawable="@drawable/btn_minus_default" />
</selector>
Parsed in 0.011 seconds, using
GeSHi 1.0.8.4
This is places in your drawable resources directory and you'll specify it as the button's image src.