We live in the mountains. Almost by definition, that means that where we are trying to grow things involves some sort of slope. Some of them are gentle, others are steeper. This particular post will discuss planting on a gentle slope (see the article on the Mountain Gardening website on dealing with steeper slopes and planting retaining walls).
If you are planting on even a gentle slope, it is a good idea to build a shallow shelf or basin to hold water. Otherwise, most of the water goes rushing down the slope and won't penetrate to the roots.
Here's a picture of what I mean: