The first thing that you should consider is the qi of the land.
You should look for land that with rich soil that support life. An example is land with lush green vegetation. You should avoid those with thin, sparse vegetation or that is yellowish in color which indicate weak or insufficient qi in the soil.
You should also avoid land that is dry, sandy or has a rocky terrain as well as stay away from waterlogged areas.
Another indicator of the presence of life supporting qi is the presence of rich fauna like earthworm, birds, squirrels etc.
Profile of the Land
A flat terrain is acceptable in Feng Shui but it is better if the land at the back of the house is higher that at the front. It is less auspicous but still acceptable if the land at the land is slightly lower but not if it slope steeply downwards.
As a general rule it is ideal if the house sits on land that is higher or the same level as the road. Try to avoid a house that sits on land that is lower than the access road.
What about the profile of the land on the left and right side of the house? The left side (looking out of the facing of the house) is known as the Green Dragon side. Classical text recommends a stronger Dragon which implies prosperity and harmony. A mountain or taller structure such as a building can act as the Green Dragon. The right side is known as the White Tiger. Classical text recommends a weaker White Tiger which is associated with disharmony and losses! A land that is flat on both the left and right side is acceptable.
Shape of the Land
In Feng Shui, a square or rectangular shaped land is auspicious. The ratio of the length to width of the land should not be too large which would result in a long strip. This is not auspicious. If possible the ratio of the length to width of the land should not be more than 2.5:1.
Irregular shaped land with missing corners are inauspicious and you should avoid them. The worst shape is a triangle. According to classical text, triangular land are highly inauspicous!