Ann Logan

As you stitch cross stitches, the floss tends to twist and eventually knot. The simple solution is to frequently dangle the needle and floss allowing it to untwist. Once you get into a routine, you will do this without thinking. Until you do, consciously take this step every few stitches.

I assume that you yourself understand this, but still I will say. It is very important to wash your hands before touching the fabric or thread. Of course, you are going to wash the finished cross stitch design, but why make it dirtier?
In case you use a thread or fabric that is hand-dyed wash them in advance so that they do not stain your handwork.
Unwind your thread from time to time. Just leave the needle and let the thread loose - it will unwind itself... Or help it a little to unwind.
Do this every few stitches, otherwise the texture will be uneven and the fabric will show through your stitches too much.