Right now, garden centres and supermarkets are full of French an African marigolds, lobelia, busy lizzies (which will flower happily in shade), begonias and other bedding plants. In a normal year, it would still be too early to consider planting these tender individuals out, but with this spring's hot, dry weather the soil is getting nicely warmed up.
Buy the best quality plants that you can find, don't be tempted by those displayed in draughty or cold areas outside shops. Avoid plants which are already in flower, they have been artificially brought to maturity and will tire quickly after a short flush of blossom. Look for full and hearty plants: when transplanted these will make strong growth and have a longer flowering period.
If the plants have not been hardened off, do so by leaving them outside during the day and bringing them in at night, for about a week. Plant out in a fertile soil - add garden compost or well rotted manure - and water in after planting. Dead head - where possible to encourage a long display of colour.