If your flowerbed is in direct sunlight for the majority of the day, you will want to choose a hardy, heat-resistant plant. Some of the better choices for full sun are marigolds, zinnias, petunias, and zinnia. Be careful to choose a variety of heat-resistant plants, as some of these full-sun varieties are prone to diseases, especially marigolds. You will want several types of flowers in your flowerbed not only for contrast and interest, but in the event that one variety does poorly or becomes diseased, you can simply pull the plants (roots and all) and cultivate your remaining varieties. Keep in mind that if any of your plants are diseased, you should pull them immediately to avoid spreading the disease to your other plants.
For areas that are in partial to full shade, consider impatiens, begonias, and vinca. All of these come in a variety of colors and grow quite well in shady areas. In choosing the colors for your flowering plants, remember that red colors make the plant seem closer and larger, while blue will give the illusion of smallness and distance. Planting a single color of flowers will draw more attention to your flowerbed, but if you want several colors make sure the flowers compliment one another. Purple goes well with pink and white will blend in nicely with any other color. Also, red goes very well with violet as odd as this may seem.
The most important thing you can do to ensure beautiful summer flowers is to prepare your flowerbed appropriately before planting. Till the soil thoroughly and remove any weeds, roots and all. Fertilize your flowerbed before planting and for the first month after planting flowers, water every other day so that the roots of the plants will be properly nourished. Use mulch as a covering in your flowerbed to preserve moisture and do not over-feed your flowers. Use fertilizer exactly as directed on the package and avoid getting the fertilizer directly on the parts of the plant that are above the ground.