Always consider where the water will go when planning a walkway or retaining wall. Proper drains are a must. Extend the life of your wood or stone retaining wall. Professional tips for retaining walls & walkways.
Keep the garden vertical. That's why it's good to look for shrubs and trees that max out interest as they grow up, not out. Try dwarf varieties for a small backyard, as well as more columnar evergreens (bonus—they boost wintertime interest, too).