As you can see, there is not one perfect tension for your yarn. It all depends on the project and the knitter. Keep in mind:
1. Try a few different needles of different material to get the gauge and tension that is best for your project. If your stitches are sliding off the needle willy-nilly, you might need to go down a size or two. If your stitches just won't budge, try a smoother material like aluminum.
2. Keep a loose grip on the yarn as you knit. Don't tug on the yarn as you complete each stitch. If you'd like your fabric to be more dense, go down a needle size or two rather than trying to knit tighter.
3. Maintain even tension by consciously focusing on your knitting rhythm for the first few minutes after you pick up your needles. Knitters who like to watch movies or listen as they knit find their tension getting too tight during a chase scene or too loose during a song and dance number. Commuter knitters find their tension changes from the ride to work and their ride home. Unless you like your knitting to be a diary of your day, pay attention to the way you begin to knit; that will keep your hands focused even while your eyes wander.
Wondering how to measure your test swatches? Maybe our Swatch Tutorial can help!