Asubha (the unbeautiful, or just loathsome) is contemplated in a famous and somewhat notorious (in the West) form of Buddhist meditation which takes this as its name. In order to counteract an unhealthy attachment to the beautiful (subha), one contemplates the unbeautiful. How does one do this? Most commonly, one concentrates on and imagines the process by which the beautiful thing one is attached to will die, disintegrate, and fall apart. This is often practiced, therefore, in cremation grounds, or in specially cultivated areas where donated corpses are left to fall apart through the action of animals and time.
The practice of asubha meditation can be shocking and quite painful. The dedicated practice of asubha typically takes a very long time.
Want to start with an asubha-nimittam (a sign or object of unbeauty)? Try this youtube video. Warning: probably not safe for work, and may be shocking to your sensibilities.