If we perceive a beautiful person, a beautiful object, desire and attachment arise, it is helpful to reflect on the Dukkha of desire and what the Four Noble Truths are. If you don't know what the Four Noble Truths are, then you had better learn them because they are the essence of this practice. Dukkha exists; the cause of Dukkha lies in our own ignorance, our craving, our desires, our attachments; that there is the ability to end Dukkha through the development of wisdom and the letting go of attachments and craving; and the fourth Noble Truth is the Noble Eightfold Path, all the techniques to get out of Dukkha.
So if we reflect on the fact that desires and attachments create Dukkha, this is important, this can help us to let go. Not just reflecting on impermanence as the person said but reflecting on the Dukkha of attachment and craving.
Now it's possible to see something that is beautiful, to appreciate the beauty but not have attachment towards it. A sunset is a clear example when reflecting on impermanence can work. When we're watching a sunset, we're watching the colors and the forms. It is quite a show, nature is putting on a beautiful color show and we can reflect that this is pleasant for the eye. It is interesting, it is beautiful, but at the same time we reflect, "Well it is just passing, it is not going to be there, it is changing right in front of my eyes," therefore we don't get attached. We know that we cannot hold onto a rainbow or a sunset or any kind of beautiful sky, we know that we can't hold onto that. So impermanence helps quite a bit.
To reflect on Dukkha also can help. That even if we could hold the rainbow or the sunset, even if we could hold onto that, so what? It is just Dukkha, in the sense of wanting, wanting. To reflect on the Dukkha of wanting can help us not to get caught up in desire and attachment.