Greywater is water that is being reused. There are many expensive systems out there which allow you to reuse water, but here is a free way to save some water.
Showering uses tons and tons of water, so first off if you want to help conserve water shower with a purpose. I know its nice to just sit under a hot shower for a couple minutes, but it wastes so much. Anyways one of the best and easiest ways to collect grey water is simply to plug up your bathtub while you take a shower. This water can be stored in a large bucket by your toilet and used in place of flushing. There are two ways of doing this, the first is to open up your tank and pour it in. This is however not suggested because the soaps and whatnot can cause damage(though I am not sure how) instead you can simply pour straight into your toilet. Believe it or not folks this works. It takes more water than it would if you flushed the toilet, but seeing as how this water is all recycled it is all possitive.
I have a friend who has his sink drain into a 5 gallon bucket instead of sending it to the sewers. He is very particular about what is used to wash up with in the sink; he only uses a garden safe handsoap to wash up with. Daily he takes his bucket and he waters his garden with it. At first I thought it was a terrible idea, but then I started looking at the soap and it at least claims that you can pour it straight on plants with no negative effects. His plants seem to be thriving so I have to believe it. You can still use this type of greywater without using natural(bizarre smeling) soap and watering your plants with it. You can instead use this for flushing your toilet to.
Those two tips will actually save a lot more water than you would think. Please leave links to other places with tips about water conservation =)