I've been slow to wrap my mind around the concept of getting gauge, but I think I am finally GETTING IT.
1. Spend some extra time knitting a gauge swatch!
2. The bigger the swatch, the more accurate the gauge results - at least 4 inches but more is better.
3. Buy extra yarn in the same dyelot in order to get an accurate gauge swatch without running out of yarn for your project
4. Knit your swatch the same way the finished piece will be knit - i.e. in the round or back and forth
5. Plan to wash and block your swatch and measure before and after
Here are some excellent articles regarding gauge:
Swatch Out! by Marilyn Roberts (Knitty)
What is a Swatch? (Stitchguide)
The Importance of Gauge (Suite 101)
Knitting Purls of Wisdom (Yarnmarket e-newsletter)
Knitting Purls of Wisdom Part Two (Yarnmarket e-newsletter - scroll down to see the notes about swatches and gauge)
And my final word about it is that for anything you intend to wear, it is absolutely critical that you do a gauge swatch! It's NOT so critical with scarves, felted bags or afghans, but it is essential that you swatch if you want the finished product to fit you. Educate yourself about how yarns behave after they are washed and blocked. Swatching can be fun because it gives you the opportunity to play with your yarn before you tackle the project.