Relative Layout. Between 2 items
You will need to add another layout so that the button will be exactly in the middle. For example, add this to your layout:
<FrameLayout
android:layout_width="wrap_content"
android:layout_height="fill_parent"
android:layout_above="@+id/button_bottom"
android:layout_alignLeft="@+id/button_center"
android:layout_alignRight="@+id/button_center"
android:layout_below="@id/button_center" >
<Button
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="center"
android:text="new" />
</FrameLayout>