Most annuals benefit from applications of organic mulches to retain moisture in the soil and smother weeds. Grass clippings, shredded leaves or bark, compost, and other organic materials also improve the soil as they break down.
While some annuals are drought-tolerant, most need plenty of water. If the soil dries out due to lack of rain, it's important to thoroughly soak the soil when you water, not just wet the surface. It's also important to keep the foliage and flowers as dry as possible to prevent disease. Soaker hoses and drip irrigation do this best. If you use sprinklers, run them in the morning so that the plants dry quickly in the sun. Watering individual plants by hand takes patience to supply enough water to thoroughly soak the soil.
Some annuals respond well to pinching -- removing the growing tips by pinching off the small, developing leaves at the ends of stems. This forces more lateral growth, making the plant bushier and shorter.
Some tall annuals tend to fall over, especially when they are heavy with flowers. To keep them upright, you can locate them so other plants help support them, or back them up against a fence or other structure and fasten them with twine. Another alternative is to insert stakes of wood, bamboo, or other unobtrusive material in the soil adjacent to the plants while they are still small and, as they grow, fasten the plants to the stake. Other commercial products are available to support plants.
Many annuals benefit from removing flowers once they begin to fade. A weekly walk through the garden deadheading spent blooms will keep many annuals flowering longer and more profusely. Some will stop blooming and die if not deadheaded. A few need to be cut back severely in midseason to encourage a new flush of growth and flowering in late summer and fall. Some annuals readily self-seed. If you want to prevent them from doing so, you need to deadhead faithfully. A few annuals, such as begonias, do not benefit from deadheading.
Most annuals need fertile, well-drained soil for healthy growth. That's why it's important to incorporate organic matter when preparing beds. Some soils may also benefit from incorporating granular fertilizer before planting. (Check your soil test results to see how much you need.) Slow release sources of nitrogen applied at planting can meet nitrogen needs for the entire season. If annuals become short of nitrogen, often indicated by yellowing of younger leaves, you can sidedress granular fertilizer or apply liquid fertilizer.