Build retaining walls to create terraces; connect the terraces with zigzag pathways. Before construction on steep slopes, it's a good idea to review your plans with a structural engineer.
Install jute erosion-control netting before planting on steep slopes. Unfurl the rolls on the slope across the grade; secure them to the ground with U-shaped galvanized or plastic-coated pins (usually sold with the jute). Cut small, X-shaped holes in the jute and plant the seedlings or plants through them.
Choose plants with dense, strong roots that help hold the soil. Examples: artemisia, ceanothus, cotoneaster, creeping mahonia (M. repens), ice plants, juniper, rockrose, rugosa roses.
Arrange plants in staggered rows when setting them on a slope where erosion may occur.
Install drip irrigation so plants get the amount of water they need without a lot of runoff. Place emitters uphill of the plants.
To catch rainwater, build berms on the downhill side of the plants using soil from the planting holes.