Experiment to find the right tension.  Your yarn should feel comfortable and slide easily along the needle. You should not have to struggle, break into a sweat, or break a needle in a effort to make one knit stitch.

A good place to start is with the recommended needles for the pattern and the yarn.  We  used Blue Sky Alpaca Sport Weight for this experiment, whose label recommends US size 3 - 5 needles for a gauge of 5 - 6 stitches per inch. We made four swatches with four different size needles, cast on 20 stitches for each swatch, and came up with very different results. 

 These needles are smaller than average for this yarn; they yielded a tight, dense fabric that continued to curl even after blocking. This fortified fabric is suitable for mittens and other projects that keep out the cold, but would not be recommended for sweaters, scarves or other projects that would benefit from a fabric which drapes and breathes.

Gauge = 6 stitches per inch

Finished measurements:  3.25 x 2.75 inches 

Swatch 2: US size 5 Lantern Moon ebony needles 

The size 5 needles yield comfortable, even stitches that hardly needed blocking.  This is a desirable fabric for sweaters or other medium weight projects.

Gauge = 5.5 stitches per inch

Finished size = 3.75 x 3.5 inches 

Swatch 3: US size 8 Addi Turbo aluminum needles 

Larger than recommended, these smooth aluminum needles glide through the yarn making a loose fabric.  Scarves and shawls benefit from a drapey, but not quite lacey stitch like this. However, this fabric would be too drapey for a sweater because it would not hold its shape.

Gauge = 4 stitches per inch 

Finished size 4.5 x 4 inches 

 Swatch 4:  US size 10 Lantern Moon ebony needles

More loose and open than the previous swatch, this swatch knit up quickly at almost half the recommended gauge.  The stitches are very open, which allows the alpaca fibers to somewhat fill the spaces between each stitch.  This fabric would be suitable for a scarf or a shawl.

Gauge = 3.75 stitches per inch

Finished size = 5 x 4.5 inches