The hand-sewing stitches I use most often are back stitch, runningstitch,and running back stitch.  Since I sew by hand only where there is a problem. I use the back stitch most, -- it is the strongest, and has the most stretch. There are two varieties: in the "open" or "spaced" backstitch, you bring the thread up the length of two stitches beyond the previous place where you came up, then put the needle down one stitch-length behind the place where you came up. This is about the only way you can sew very thick fabric without resorting to the picky-embroiderer's tedious stab-straight-down-stab-straight-up.  Since the nip you take though the thick fabric is three times the length of the stitch you are making, it is easier to make stitches that are short enough. 

The closed back stitch is the same except that you bring the thread up only one stitch length beyond, and go back down in the same hole where the previous stitch came up. This is useful for replacing machine stitching through the original holes -- almost the only way you can mend frayed top-stitching invisibly.   The running stitch goes down and up at equal intervals. It can be done by weaving the needle in and out several times before pulling it through.  Light fabrics can be pleated onto the needle to make the work go faster.This makes the thinnest line of stitching, and is the stitch you should use to seam sheer fabrics. 

In the running back stitch, you *weave the needle through the fabric as many times as you can, pull the thread through, put the needle down one stitch length behind the place where you last came up, repeat from *. This goes nearly as quickly as running stitch, but the back stitches give it a little stretch, and stop rips from propagating.  I never use a doubled thread unless I need heavy-duty thread and haven't any. (I also use a double thread when the only available thread is of doubtful quality.) Double threads must be watched to see that they don'tget out of sinc, so it's easier to sew with one heavy thread than with two thin threads. Beginners may sew with doubled thread made by putting the needle in the middle of a double-length thread, but you should switch to a single thread,or to two separate threads put through the eye together, as soon as you can-- it's much easier to sew if you can adjust the length of the thread, the thread tends to wear through if the fold never shifts, and it is difficult to remove kinks and tangles from a thread that's anchored at both ends.It is best to keep the tail only a little shorter than the thread until the thread gets uncomfortably short, because frequent shifting keeps the thread from fraying at the needle.When basting, knot the end of the thread by wrapping it around a finger and rolling it off. The knotted end is easier to find and remove. When stitches are meant to stay, start with a few back stitches to secure the end of the thread. Knots tend to pull through or wear off, and are not secure in anything that is going to be worn and washed. A beginner may knot the end of the thread, take a few closed back stitches, and then shear off the knot.