I recently read this from a crochet Yahoo group called 'crochet wearables':
If you want your garment to fit properly after it is completed, it is important to take the time to make a swatch. Then treat it exactly as you will treat your finished garment.
First, chain about 20 stitches (staying in pattern may require you to start with a few more than 20 chains).
Work your pattern until it measures about 4 or 5 inches.
Lay it flat and let it rest.
Measure the length and width and write these measurements down.
Wash and dry the swatch the same way you will be washing and drying your finished garment.
Measure the length and width of the laundered swatch and write these measurements down.
Determine how much (if any) your swatch has changed from before it was washed and dried. This will help you figure how to adjust your pattern to fit properly. You may need to make your garment much larger if your swatch shrank a lot. Or you may need to make your garment smaller if your swatch grew (stretched).
Mark out a 4 inch square in the center of the swatch. You can do this by pinning the 4 outermost corner stitches of the square.
Count how many stitches and rows are contained in those 4 inches. This will help you determine how many stitches and/or rows to add to or subtract from your pattern in order for your garment to equal the correct finished measurement.
You should do this every time you begin a new (large) project, even if it is the same pattern with the same brand of yarn and the same hook size.
( Personally, I'd be more concerned about all this in regards to wearables.)
Sometimes different colors of the same yarn brand will work up to different gauges. Sometimes the yarn companies change something in the fiber content or manufacturing process of their yarns. And sometimes your tension will be tighter or looser than it was before. A lot of things could make your tension change your gauge, such as whether you are happy, sad, angry, tired, rushed, very relaxed, etc.
So, start a small piece with a seperate ball of yarn with the same hook for a few minutes, just to get your hands warmed up, and your muscles relaxed before going back to your main project. This is good when you plan to go back to a earlier project you haven't worked on in awhile. ;-)
If you do a gauge swatch before you start your project, there shouldn't be any unpleasant surprises.
So, get your 'Crochet On'!