A little extra care will keep your annuals looking attractive all season long.
Though most annuals are bred to be compact and well branched, some, including petunia and geranium (Pelargonium), benefit from pinching. With your thumb and forefinger, nip off the tips of the stems; this forces side growth, making the plant denser and more compact.
A few very tall annuals, such as sunflower (Helianthus), may require staking to keep them from falling over. Use a single stake and tie the stem to it with plastic tape or soft twine; or support the plant with a cylinder of wire fencing.
As their flowers fade, annuals put their energy into ripening seeds. If you regularly "deadhead" - remove dead flowers and any seedpods - the plant will typically bear more flowers in a continued effort to produce seeds. Deadheading also keeps the garden tidy.
Large-blossomed annuals such as zinnia are easy to deadhead: just pinch or cut flowers back to the next branch. With smaller-flowered plants like floss flower (Ageratum), it's easier to shear the flower heads off with pruning or hedge shears. Take care not to cut back too far: don't remove more than a third of the plant.