You can read up on the basic principles of design in many books and websites (unity, balance...) , so I won't bore you with them here. However, here are some tips that I think are most important to help take your garden to the next level.
- don't forget to plant trees and shrubs for structure and winter interest,
- don't plant just one of everything - use groups of three's and five's or more,
- arrange plants in small "vignettes" (three or four plant types that work well together),
- group plants together with contrasting foliage colour, shape and size for an interesting scene even when they're not blooming,
- repeat a few key plants throughout your yard or garden for continuity.