Save list view in android app

I made a custom list with an edit text and add button. The user inputs text into the edit text, hits the add button and it adds the button to a list view. The problem is, once I exit the app all the lists added got deleted. How do I easily save the list view so when I open the app the items added don't get erased.

activity_main:

<RelativeLayout
xmlns:android="http://ift.tt/nIICcg"
android:layout_width="match_parent"
android:layout_height="match_parent" >

<EditText
    android:id="@+id/editText"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_alignParentTop="true"
    android:layout_toLeftOf="@+id/addItem"
    android:hint="Add a new item to List View" />

<Button
    android:id="@+id/addItem"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignParentRight="true"
    android:text="Add" />

<ListView
    android:id="@+id/listView"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_below="@+id/editText" >
</ListView>

item_row:

<RelativeLayout
android:id="@+id/relativeLayout1"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
xmlns:android="http://ift.tt/nIICcg"
android:padding="5dip">

<ImageView
    android:layout_width="78dip"
    android:layout_height="78dip"
    android:id="@+id/imgThumbnail"
    android:scaleType="centerInside"
    android:layout_alignParentTop="true"
    android:layout_alignParentLeft="true"
    android:layout_alignParentStart="true">
</ImageView>

<TextView
    android:textAppearance="?android:attr/textAppearanceLarge"
    android:layout_height="wrap_content"
    android:text="TextView"
    android:layout_width="wrap_content"
    android:id="@+id/txtTitle"
    android:layout_toRightOf="@+id/imgThumbnail"
    android:layout_marginTop="6dip"
    android:layout_marginLeft="6dip">
</TextView>

<TextView
    android:layout_height="wrap_content"
    android:text="TextView"
    android:layout_width="wrap_content"
    android:id="@+id/txtSubTitle"
    android:layout_toRightOf="@+id/imgThumbnail"
    android:layout_below="@+id/txtTitle"
    android:layout_marginTop="3dip"
    android:layout_marginLeft="6dip">
</TextView>

MainActivity:

public class MainActivity extends ActionBarActivity implements  
View.OnClickListener{

EditText et;

Button bt;

Button rbt;

ListView lv;

List<ListViewItem> items;

CustomListViewAdapter adapter;

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    et = (EditText) findViewById(R.id.editText);
    bt = (Button) findViewById(R.id.addItem);
    bt.setOnClickListener(this);

    rbt = (Button) findViewById(R.id.removeItem);
    rbt.setOnClickListener(this);

    lv = (ListView) findViewById(R.id.listView);
    items = new ArrayList<ListViewItem>();
    adapter = new CustomListViewAdapter(this, items);

    lv.setOnItemLongClickListener(new ListView.OnItemLongClickListener() {
        // setting onItemLongClickListener and passing the position to the function
        @Override
        public boolean onItemLongClick(AdapterView<?> arg0, View arg1,
                                       int position, long arg3) {
            removeItemFromList(position);

            return true;
        }
    });
}

@Override
public void onClick(View v) {
    switch(v.getId()) {

        case R.id.addItem:

            items.add(new ListViewItem() {{

                ThumbnailResource = R.mipmap.ic_launcher;
                Title = et.getText().toString();
                SubTitle = "Item2 Description";

            }});
            lv.setAdapter(adapter);
        break;
    }

}

class ListViewItem {

        public int ThumbnailResource;
        public String Title;
        public String SubTitle;
}

protected void removeItemFromList(int position) {
    final int deletePosition = position;

    AlertDialog.Builder alert = new AlertDialog.Builder(
            MainActivity.this);

    alert.setTitle("Delete");
    alert.setMessage("Do you want delete this item?");
    alert.setPositiveButton("YES", new DialogInterface.OnClickListener() {
        @Override
        public void onClick(DialogInterface dialog, int which) {
            // TOD O Auto-generated method stub

            // main code on after clicking yes
            items.remove(deletePosition);
            adapter.notifyDataSetChanged();
            adapter.notifyDataSetInvalidated();

        }
    });
    alert.setNegativeButton("CANCEL", new DialogInterface.OnClickListener() {
        @Override
        public void onClick(DialogInterface dialog, int which) {
            // TODO Auto-generated method stub
            dialog.dismiss();
        }
    });

    alert.show();

    }
}

CustomListViewAdapter:

public class CustomListViewAdapter extends BaseAdapter {

LayoutInflater inflater;
List<MainActivity.ListViewItem> items;

public CustomListViewAdapter(Activity context, List<MainActivity.ListViewItem> items) {
    super();

    this.items = items;
    this.inflater = (LayoutInflater)context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
}

@Override
public int getCount() {
    // TODO Auto-generated method stub
    return items.size();
}

@Override
public Object getItem(int position) {
    // TODO Auto-generated method stub
    return null;
}

@Override
public long getItemId(int position) {
    // TODO Auto-generated method stub
    return 0;
}

@Override
public View getView(final int position, View convertView, ViewGroup parent) {
    // TODO Auto-generated method stub

    MainActivity.ListViewItem item = items.get(position);

    View vi=convertView;

    if(convertView==null)
        vi = inflater.inflate(R.layout.item_row, null);

    ImageView imgThumbnail = (ImageView) vi.findViewById(R.id.imgThumbnail);

    TextView txtTitle = (TextView) vi.findViewById(R.id.txtTitle);

    TextView txtSubTitle = (TextView) vi.findViewById(R.id.txtSubTitle);

    imgThumbnail.setImageResource(item.ThumbnailResource);

    txtTitle.setText(item.Title);

    txtSubTitle.setText(item.SubTitle);

    return vi;
   }
}