If you're leaving town for a while but don't want your plants to wither while you're gone, you can ask a friend to stop by and water them, or you can build a system that dispenses just the right amount of water for each plant, automatically. All you need are some plastic baggies, a needle, and some thread.
Instructables user jjdebenedictis threads a very fine needle, and then punctures the bottom of a zip-loc bag with the needle. The needle has to be straight and fine for this to work, and once punctured the thread hangs out of the bag. Knot the thread so it stays in place, and remove the needle. 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, and 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.
Of course, if this is too much work, this soda bottle method also works, and if you want to get high tech, you could build this arduino based system. Either way, hit the link below to see how this one's done.
Cheap, Easy, Leak-Safe Way to Water Plants While On Vacation | Instructables