First of all, the most important thing you can do is to plant annuals in locations that match their need for light and water. You wouldn't thrive in the wrong location, and neither will your annuals.
Follow the retailer's instructions to plant the annual. To increase the size of your annuals later in the season, pinch off any blooms on young plants. Do this for three weeks after you plant the annual. This will cause the plant to focus on growing, not blooming, and you will be rewarded with larger plants and many more blooms later in the growing season.
In general, annuals planted in the sun will respond well to a good drink on a hot day. But be careful to give them individual drinks of water if they are cross-planted in a bed with permanent plants that have different water needs. It is a little more time consuming that a quick spray with the hose, but won't damage your permanent plantings. Sprinklers can get flowers too wet and ruin their colors. Soaker hoses or individual waterings are best.
Keep mulch two to three inches away from flower stems and keep it loose throughout the growing season to give your annuals room to spread and grow larger.
