Android : Android - how to stop background service?

I am beginner in Android development and now learning services. I have created a simple service to toast a string continuously. I took two buttons to start and stop service. When I am clicking on start button, the service is started and It is toasting string continuously. But when I go to stop service by clicking on stop button then it is not getting stopped. It is still toasting a string continuously. Thanks.

public class MyService extends Service {

    @Override
    public IBinder onBind(Intent arg0) {
        return null;
    }

    @Override
    public int onStartCommand(Intent intent, int flags, int startId) {
        Toast.makeText(this, "Service Started", Toast.LENGTH_LONG).show();

        doToast();

        return START_STICKY;
    }
    @Override
    public void onDestroy() {
        super.onDestroy();
        Toast.makeText(this, "Service Destroyed", Toast.LENGTH_LONG).show();
    }


    public void doToast()
    {
        final Handler handler= new Handler();
        handler.postDelayed(new Runnable(){

            @Override
            public void run() {
                // TODO Auto-generated method stub
                Toast.makeText(MyService.this, "hi", 0).show();
                handler.postDelayed(this, 3000);
            }

        }, 3000);

    }

}

This is code to start and stop service

startservice.setOnClickListener(new OnClickListener() {

            @Override
            public void onClick(View v) {
                startService(new Intent(getBaseContext(), MyService.class));
            }
        });
        stopservice.setOnClickListener(new OnClickListener() {

            @Override
            public void onClick(View v) {
                stopService(new Intent(getBaseContext(), MyService.class));
            }
        });