Plant the late show. Planted now, many annuals will bloom into fall, so don't feel like the opportunity for summer color has passed you by. Fill holes in solid beds of annuals, or fill gaps in the perennial garden where spring-blooming kinds have died down or been cut back. Just make sure to water individual new plants every day, or even twice a day if necessary, until they get their roots into the surrounding soil (this may take several weeks).
Some that work particularly well at filling late-season holes are the tall gloriosa daisies and the equally tall echinacea, or the much lower and even more versatile cream-or green-flowered petunias. These small-flowered petunias seem to go with anything and cover ground between other plants. Other annuals that will last into fall include alyssum, celosia (especially the nearly wild plume types), cosmos, golden fleece, marigold, portulaca, red or blue bedding salvia and vinca. Zinnias love the heat of late summer, but may mildew in fall.
* Irrigate early. You've probably heard that's it's a good idea to water early in the day so foliage is dry by nightfall--the practice helps prevent disease. Recent research has found that watering in the morning also cuts down on slug damage because slugs need a moist soil to travel over at night.