Building raised planting beds and retaining walls are good do-it-yourself projects.
A lot of people use railroad ties or treated landscape timbers for this type of landscaping, but you really have to install them properly, or you'll end up with rot problems.
For do-it-yourselfers, we think interlocking concrete blocks work well. There are a few different brands on the market today, but they all work on the same principles:
Preparing the Foundation
Probably the most important part of installing interlocking concrete blocks is to prepare the foundation for the wall properly.
You want to make sure that as you dig out the area, you don't disturb the soil that you're going to leave in place. This is called "skimming."
As you dig out, check to make sure that the base is level.
You want the base low enough so that the first row will sink below ground level on the front. As a rule of thumb, you should bury the bottom row about 1" for every 8" of wall height above the ground.
For larger projects dig down about 6 inches below where you want the wall to start, and then put in some class 5 fill.
Compact the fill with a 4 x 4 or hand tamper. Again, make sure it's level all along the wall.
For really large retaining walls on a large hillside, water drainage is usually a problem because it'll flow down the hill and start eating away at the fill behind the retaining walls.
To eliminate this problem you need to lay a moisture membrane behind the wall to catch the water, along with some drain tile to collect the water and drain it away.