How effective the powa is depends on how well one practices the Five Powers at death. And this depends on how well one does the general practices in the life. The Lam-rim explains the four ways to accumulate powerful, extensive merit. One should attempt this in every day life, while doing all the normal activities, eating, sleeping and walking, etc., rather than the actions being done out of worldly desire, samsaric attachment. There should be beautiful views, beautiful art, flowers, Tara images.
Flowers give a very special spiritual feeling.
The point is to create a positive imprint on the person's mind. By being there, the person is not afraid of dying, their mind is elevated. When families come, they see it's a nice place and want to come, a place that makes it feel there's no need to be afraid of death. Because of His Holiness the Dalai Lama's wisdom and compassion, more and more people are feeling comfortable with Buddhism these days. We need to help people have an open mind to different methods.