Android custom button margin

I have a custom button layout

<?xml version="1.0" encoding="utf-8"?>
<selector xmlns:android="http://schemas.android.com/apk/res/android">
    <item android:state_pressed="true">
        <shape>
            <gradient android:startColor="@color/pres1"
                android:endColor="@color/pres2" android:angle="270" />
            <stroke android:width="5dp" android:color="@color/stro3" />
            <corners android:radius="5dp" />
            <padding android:left="10dp" android:top="20dp"
                android:right="10dp" android:bottom="20dp" />
        </shape>
    </item>
    <item android:state_focused="true">
        <shape>
            <gradient android:endColor="@color/focu1"
                android:startColor="@color/focu2" android:angle="270" />
            <stroke android:width="5dp" android:color="@color/stro2" />
            <corners android:radius="5dp" />
            <padding android:left="10dp" android:top="20dp"
                android:right="10dp" android:bottom="20dp" />
        </shape>
    </item>
    <item>
        <shape>
            <gradient android:endColor="@color/norm1"
                android:startColor="@color/norm2" android:angle="270" />
            <corners android:radius="5dp" />
            <padding android:left="10dp" android:top="20dp"
                android:right="10dp" android:bottom="20dp" />
        </shape>
    </item>

</selector>

And for the below lay out

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical" android:layout_width="fill_parent"
    android:layout_height="fill_parent" android:background="@color/all_white">

    <TextView android:layout_width="fill_parent"
        android:layout_height="wrap_content" android:text="@string/hello"
        android:textColor="@color/all_red" />

    <Button android:id="@+id/mq_categories" android:layout_width="fill_parent"
        android:layout_height="wrap_content" android:text="Browse Quiz Categories"
        android:background="@drawable/custom_button" />
    <Button android:id="@+id/mq_random" android:layout_width="fill_parent"
        android:layout_height="wrap_content" android:text="Enter Random Quiz"
        android:background="@drawable/custom_button" />

</LinearLayout>

following output is generated

I need to add some margin between buttons, Any help appreciated..