If you have a greenhouse or lean to and can shelter plants under glass, a good tip is to plant up a hanging basket or tub and grow on in the greenhouse. This gives the plants time to settle and establish roots in the container. Bedding plants grown under glass will need to be "hardened" off before planting out. This means getting the plant accustomed to the outside conditions and not moving it straight out from the calm, still and warm greenhouse conditions to the less perfect outside weather. Move the trays of bedding plants outside, starting on milder days and leaving out for longer as the weather improves until they are out all the time and fully accustomed to our weather.
Bedding plants are also attractive to slugs, some more than others. For example, Marigolds are slug magnets, but Calendula are not. Protect with slug protection such as pellets or slug traps. Check out Best Bedding plants for ideas on which bedding plants are slug resistant and which are low maintenance.
Bedding plants have to grow a lot and produce lots of flowers quickly. It follows they do best in good compost and will always benefit from feeding during the growing season.
Dead heading is also essential to keep bedding plants flowering. In summer when in full bloom, dead heading may need to be done several times a week depending on the plant. Some bedding plants are easier to grow and easy to grow than others. For example, Mesembryanthemum common name ice plant is a fabulous showy bedding plant, bottom left, but look at the dead heads in the image next to it removed in just one day. If a plant is not dead headed, it will cease to flower, but taking off 50+ spent flowers is very time consuming. Compare that with a Pelargonium, (image 3 top right) which needs much less dead heading and Nasturtiums, 1st image top left, hardly need any attention.