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).
Juniperus communis 'Stricta' Blue spruce 'Maigold' The rocky juniper 'Blue Arrow' Blue spruce 'Glauca Compacta' Deren white 'Aurea' Yew ‘Washingtonii’ The European larch 'Pendula' The average juniper 'Gold Star' Rough fir 'Compacta' Oregano 'Thumbles'
*Boxwood 'Dee Runk' is reportedly the nicest of the columnar boxwoods tolerating shade and clay soils. Total height of ft tall can be sheared shorter. Max width about two feet. Advantage over other upright boxwoods is dense growth that won't flop open.