The lessons from Piet's designs are many, but my favorite and most easily adopted is drifted planting. Instead of planting 3 or 5 of something, he plants (where there is room) 9 or 10 or more. In this way, plants that are perhaps slightly common become stellar. These gardens also embrace a sense of messiness and lack formality that is often hard to maintain.
The other tidbit I take from these are a short list of plants that not only look good together, but they look good right now (late summer) and generally have enough interesting form to continue to look great with frost on them in the winter.
Some of the plants you see here are Echinacea purpurea 'Rubinglow', Molinia caerulea ssp. Caerulea, Salix, Allium sphaerocephalon, Salvia and Koelreuteria paniculata.
Images: Nicola Brown, Walter Herfst, Neil Holmes, Marianne Majerus, Jürgen Becker, Jo & Rob Whitworth, and Andrew Lawson via Piet's Website.