Look at this thread
on how stitches get added. Usually it's at the beginning of a row and the yarn is up over the top of the needle. This pulls up the end st and makes it look like 2 stitches. So hold the yarn a bit out to the side so you can see the leg to knit into for that stitch.
Not dropping sts comes with practice I guess, or maybe you're letting go of the right needle to wrap the yarn and it's sliding out of the stitches. When/if you let go, hold the tip with the thumb and index finger of your left hand to make sure it doesn't go anywhere.