Garden planning is mostly about being honest about how much time you'll spend, and designing for that level — not the level you wish you had.
Start with structure
Shrubs and small trees give a garden bones that look good even in winter. Plan structure first, color second.
Repeat plants
Repeating three or four key plants throughout a garden looks designed; planting one of everything looks like a collection.
Plan for bloom sequence
Sketch a calendar of bloom times across your plants so you have color every month from April through October.