If you're going on holidays and you don't want your plants to wither while you're gone, you can usually ask a friend to stop by and take care of them for you. Alternatively, you can build a system that dispenses just the right amount of water for each plant automatically. All you need are freezer bags, a needle and thread.
Instructables user jjdebenedictis threads a very fine needle, and then punctures the bottom of a zip-loc bag with it. The needle has to be straight and fine for this to work. Once punctured, the thread hangs out of the bag, but the needle rests on the inside. This way, the needle isn't in the way, and the thread serves as a wick that water can travel down. Fill the (now leaky) baggie with water, rest the baggie inside the plant's pot (or anywhere near it, as long as it's above the soil and the wick is touching the soil), and you're done.
We've shown you a similar method in the past, but it wasn't effective for multiple plants unless you buried the cotton. With this method, you can use bigger baggies (or multiple baggies) for larger plants, smaller ones (or less filled) for plants that won't need much water. As long as the thread is in contact with the soil, you've got a self-watering plant that only takes as much water as the soil can handle.
Cheap, Easy, Leak-Safe Way to Water Plants While On Vacation [Instructables]