If you are only using your container for a temporary display — for example, summer bedding and winter/spring bedding (not hanging pots for the latter), a good soil-less compost will be quite adequate. In a reasonably large tub, or whatever, you can usually get two consecutive plantings from one fill-up of compost — say, a spring bedding scheme of wallflowers and bulbs followed by a summer one of geraniums, French marigolds, lobelia, etc. After this the compost becomes rather ‘played out’ and full of old roots, so it is better changed. Small pots, baskets, tubs, etc., need new compost every time they are replanted.
Tubs, troughs, large window boxes and the like intended to be used for permanent plantings are best filled with a John Innes-type, soil-based compost. It is not necessary to put drainage crocks at the bottom of containers if they have adequate drainage holes.
Always remember to fill the container, if it is a large one, after you have positioned it in its final place!
It is difficult to lay down hard and fast rules about how the tub or whatever should be stocked, and a list of suitable plants for particular types of container I will post on Monday, but generally speaking, taller ones look better with taller plants in them, although shorter ones, and trailing things, can be used around the edge. The tallest subjects should be in the centre of a container designed to be viewed from all sides, or at the back if it is placed against a wall. Flat planters look best with plants that are not too tall. In the case of a window box, dwarf varieties should always be used if there is any chance of taller ones obscuring the window. Trailing plants hanging down the front of the box or trough are most effective.
In the case of a permanent planting, it is not necessary to keep repotting but the appearance and wellbeing of the plants is improved if the top 2 or 3 in. (say 75 mm) of compost is removed annually and replaced with fresh.