I can hear the groans already... Ugh, swatching is so boring! I want to start knitting this exciting new project right now! Yes, I know you do :) But, trust me, spending an hour swatching will save you from heartache down the road.
Some of us don't mind the knitting part of swatching, but don't really want to "waste yarn" to make that swatch, especially when it's not clear whether you will end up winning at yarn chicken or not! So either we don't swatch at all, or we knit itty-bitty swatches that really don't allow for an accurate result. Ideally, you should knit swatches that are around 6 inches or 15cm in width and at least 4 inches or 10cm in length (more if your row gauge is important). "But, but, all that beautiful yarn will be wasted!!"
As it turns out, there's an easy solution for that excuse: just don't cut the yarn when you're done swatching! Make a slip knot with the yarn still attached to the ball as shown below, and then plop the yarn ball in a bowl and dunk that swatch in the sink! (yes!! swatches need to be blocked otherwise you might have bad surprises if the yarn tends to grow during blocking!)
Ta-da! The umbilical cord hasn't been cut, you see? Once your swatch has soaked for about 20-30 minutes, gently squeeze the water out of it by rolling it in a towel (being careful not to undo the slipknot), then let the swatch dry flat.
You can now take your measurements and compare your gauge to the gauge that is recommended in the pattern, which will tell you whether or not you need to adjust your needle size (the needle size in a pattern is merely given as a guideline, but each knitter knits more or less tightly and it could be that two knitters will need two different needle size to achieve the same tension.)
Once you have determined your stitch and row gauges and written it down somewhere (don't forget to also write down the needle size you used!) you can simply undo the slip knot and frog the swatch! Voila! No yarn "wasted" swatching!