Perfect Seams
Only neat, almost invisible seams finish off your knitted garment perfectly. Mattress stitch is the best method for side and sleeve seams. For joining seams use a blunt darning needle so as not to split the yarn when sewing. If possible, use the original yarn used for the garment to sew the pieces together. Instead of wicking or bobble yarn, or extremely thick yarn you can use a thinner, smooth wool in a suitable colour, or cotton (depending on the garment). Sewing thread (despite its name) is not suitable here because it is not elastic.

And here's how to do it
Mattress stitch is always done from the right side. Lay the pieces to be joined side by side with the right side facing you. Start the seam at the bottom edge. First join the cast-on rows, inserting the needle between the first and second stitch in from the edge, underneath one of the "bars" of yarn that run between the stitches. Then, working with the other piece, do the same.

On the right piece the bars are pricked up between the last and the edge stitch...	And on the left piece between the edge and the first stitch, so always working 1 stitch in from the edge.	Work the same way for reverse stocking stitch.

After 2 - 3 cm seam pull the thread fast.
With this method all patterns can be perfectly joined. In the seam the corresponding rows of the two pieces always come together, so that stripes, jacquard, rib structured and hole pattern can be ideally combined.

Divergences Allowed
In stocking stitch it is sometimes recommended to pick up 2 bars. First you combine the cast-on rows. Then you pick up the first bar on the right piece, then the 1st and 2nd bar on the left side, the 2nd and 3rd on the right side, the 3rd and 4th on the left and so on. Whether you pick up one or two bars is entirely up to you.
Try both and see what's best for you!
Sometimes it is better to take the centre of the last stitch before or the first stitch after the edge stitch for the seam. This is the case when the pattern calls explicitly for either a knit or a purl stitch (e.g. in rib stitch). Both halves of each side form one stitch.
Also if the edges are not so even as they should be (see the knitting tip "Pretty Edges") you can improve the overall result by taking the centre of the stitch for the seam.

Another Tip Neat edges are not necessarily just a means to an end, they can also be a decorative element. Decorative outer seams that can be stitched over are always an eye-catcher! If you want to emphasis these seams knit the edge stitches in a contrasting colour or in an effect yarn. Join the seams as described above, but this time from the wrong side. On the other side - the right side - there's a neat web of edge stitches.