When planting up containers, add water-retaining gel crystals to the compost to prevent unnecessary water-loss. Buying a box, rather than sachets, is far more economical: a 100 gram box (which costs about £3.60) will treat up to 100 litres of compost.
The crystals can also be added to very dry beds (the potassium-based ones break down after about three years), but a cheaper alternative is to mulch the soil surface after planting and watering well.
And, as Tom Byrne says, you must feed containers of half-hardy annuals weekly (and dead-head) to keep up a good display. Any of the proprietary soluble feeds, or tomato fertiliser, are suitable. Organic gardeners may wish to use "manure tea" (suspend a bag of farmyard manure in a barrel of water to make a nourishing liquid). For those who don't have access to the real thing, there is an all-natural, bagged product made from poultry manure available now - in some garden centres.