Blanch green veg only briefly in boiling water, then plunge them straight into a bowl of iced water to keep all of the flavour, nutrients and the fresh green colour. If you want to serve them hot, just warm through in the microwave or oven just before serving.
Simon's tip for vibrant green vegetables is to add a small pinch of bicarbonate of soda to the boiling water.
Steaming vegetables is another way to keep the nutrients inside rather than leaching out into the water. You don't need an expensive piece of equipment; simply cook your vegetables in a sieve over a pan of boiling water with a lid on top or in a bamboo steamer from an Asian supermarket. Steam vegetables until they are just tender.