Deadhead and multiply
If you have some perennials you don't want colonising the entire garden, deadhead now. Some are prolific and very keen to seed. At the same time, collect seed from those you'd like to multiply.
Cut and bulk
Take cuttings from all your favourite plants to bulk up your plantings and to replace anything that you lose over the winter months.
If you've ordered trees from mail-order nurseries, plant as soon as possible as they're often root-trimmed.
Hedge your bets
You can plant new hedges from now on, but think ahead - hedges will grow wider than you ever imagined.