Take cuttings late in the month of favorite geraniums, coleus, begonias, and any other annual flowers that you want to overwinter for replanting next year. You can also bring these plants indoors for the winter if you have a sunny spot. Several popular bedding plants are perennial in warm climates and can be brought indoors as houseplants if you don't wait until the weather gets too cool. Cold temperatures can set them back and make it hard for them to recover. Gradually move the plants into shadier locations so they are better adjusted to the reduced light levels when you move them indoors.
As you remove spent plants from your garden beds, sow a cover crop, such as winter rye. This will help reduce weed infestation, minimize erosion and compaction from fall rains, and will add nutrients and organic matter to the soil when it is tilled under next spring. If you're making a new bed, it's also a good idea to build the soil with a cover crop for a year before planting. Till in the cover crop before it goes to seed, either later this fall or early next spring.
If you go on vacation, even for a few days, make sure your plants will have a plant sitter to keep them watered. This especially is important for container plants and hanging baskets which may need water daily in hot and dry weather.
If you return from vacation to find tired plantings, you have two choices: cut them back and fertilize and hope they rejuvenate in time for you to enjoy them; or toss them and start anew (giving up is not an option yet!). You can still find annuals at more complete garden centers, or pot up some blooming perennials that you can plant in the ground later in the fall. Or consider a combination of foliage plants, such as coleus, that you can bring indoors for the winter.
If you have bare spots in the garden where early season annuals have finished flowering, or early blooming perennials such as bleeding heart have died back, consider container plantings to add color. Make a note on the calendar now, for next year, when potting annual flowers in the spring to pot some extras in large pots just for this purpose. Sweet potato vines and nasturtiums hanging over pot edges might be used, as might be the large-leaved upright and colorful cannas.