Latex paints are much easier to clean up than oil based paints because latex paint cleans up easily with water. As long as it's appropriate to use a latex based paint, choosing latex over oil will make clean up a whole lot easier.
Just like in elementary school, the sooner you clean your brush after you are done painting, the easier it will be to clean. Putting it off will lead to a damaged brush, which can be expensive in the long run. Don't put this task off if you want to keep your brush in good shape. If you don't have the option to wash your brush right away, wrap it in plastic and put it in the refrigerator, which will keep the brush fairly fresh for about a day.
Use Water Pressure
If possible, clean your brush and roller covers with a hose or faucet that has the highest water pressure possible. Higher water pressure will let you blast semi-dry bits of paint from the base of bristles in your brush. The water in our house comes out the fastest from the basement wash basin and the hose in the back yard. Check a few faucets to determine which one gives you the highest water pressure.
You don't have to throw away roller covers after one use. If you clean them properly, you can use them for several paint jobs.
Make sure the water runs absolutely clear when you are cleaning brushes and roller covers. A brush looks clean before it is actually clean. If you get all the paint out, you will be able to use the brush much longer because dried paint won't build up at the base of the bristles. Check the runoff by getting the paint bristles wet, holding the brush horizontally in front of your eyes and bending the bristles upward. The water that runs out of the base of the brush should be clear before you are done cleaning.
If you want to avoid this kind of mess altogether, it might be a good idea to hire a professional painter. If you are in the local area, there are plenty of Portland Painters that can do a great job and at a fair price!