Most plants can get by with being watered once a week if they are watered well before you leave and moved out of direct sunlight. If you're going on vacation for longer than that, or have plants with special needs--like basil, which needs to be watered frequently--then you need to either have a trusted friend "babysit" your plants or rig up an automatic watering system. You can use store-bought watering systems like terracotta cones and glass bulbs that slowly drip water into the soil, or create your own system by following the steps below.
Water each plant thoroughly and group them together in a low- or medium-light location to minimize their need for water.
Fill a water reservoir--a large bowl or bucket will do--with lukewarm water and place it above the level of the plants you intend to water while you're on vacation.
Trim a long cotton shoestring or a long cotton candle wick--they come in bundles that you can cut to size--so that it's long enough to reach from several inches inside each plant pot, across to the water reservoir, and down to its bottom. Cut a cotton shoestring or wick for each small- to medium-sized plant. For plants with heavy water needs or extra-large pots, use multiple shoestrings or wicks.
Bury the end of each shoestring or wick at least an inch deep in the pot, near the stem of the plant. If you're running additional strings into a pot to supply more water, space them evenly around the stem.
Place the other end of each shoestring or wick in the water reservoir. Cover it loosely, if necessary, to keep animals or pests from getting into it. Make sure not to pinch off the wicks with the lid; as long as one end is in the water reservoir and one end is in the pot, the wicks will continue to provide water to the plants. The larger the reservoir, the longer the water will last. But, don't forget that you'll lose some water to natural evaporation, too.