Have you ever had a sweater that you painstakingly hand knit, worn lovingly, then washed and had it turn up with little bits of fuzz or pills all over it. This article in Bellaonline Spinning explains how to start from the fleece to prevent this, but because you may not have spun your own yarn you may need sweater first aid.
One suggestion before starting your sweater is to choose wisely when selecting a yarn. A very soft yarn, if the short fibers have not been removed, is prone to pilling, this can be minimized by choosing a pattern with a knit and purl pattern that will hold the short fibers together or hide the pills a bit. It is friction that also causes the sweater to pill and if you are making a sweater or item that will be used a lot, the pilling will be more pronounced. Using the soft yarn for items that will be used less frequently, and therefore will require less handling and washing, may be an answer.
As Laun points out it is the short fiber strands that pull out from the spun yarn that create the pills on the fiber. Once a garment is knit however it is too late to choose a different yarn, therefore if you have a store bought or hand made sweater that is pilling all is not lost. One frequent suggestion is to hand wash your sweaters inside out as this will minimize the pilling, which makes sense if pilling is exacerbated by friction, there is much less friction in hand washing. Another suggestion I see frequently on the internet is dry cleaning, but I hesitate to suggest sending your hand knit items to the dry cleaners as the results vary widely depending on the cleaners.
There are sweater de-fuzzers on the market and they are sold at many stores, success with them varies depending on how carefully they are used. Some are electric, some are stones, in addition, some people use disposable razors, but before using any of them test first on an area that will not show, or better yet if you have your swatch, wash it and rub it around a bit until it pills and test that. The cons to these commercial products is that they can change the character of the sweater by giving it a felted look, or they can possibly snag the stitches on the sweater or even cut the item. The pros of using these items seem to be speed, they donít take a lot of time to use.
My recommendation, as always, is to experiment and use caution and moderation. I have never used the commercial products. Frankly, I sit with the pilled object and just pull the pills off by hand, it is a bit time consuming, but my results have been excellent. Iíve never damaged a sweater yet.