Before Stitching:
Allow enough fabric around your design for stitching. A good rule of thumb is to add 5" to the design size.
To prevent color bleed, set your thread colors by soaking them in cold vinegar water. Allow your yarn/floss to air dry.
When Stitching:
Wash your hands often and do not use lotions.
Do not leave your work stretched in a hoop when you are not stitching. Hoop marks are noticeable after your piece is stretched.
When stitching areas of the same color, do not jump more than one-fourth or one-half inch. Not only with this save you yarn/floss, it will also prevent noticeable traveling threads that can be seen from the front.
Be sure to run all loose ends under the finished work on the back side. Do not tie knots in your thread.
Work design with selvage on your right and left, not up and down (this is most important if you are making two items exactly the same size). If in doubt which is the selvage side, use the weaver's method, pull one thread from each direction, the one with the most "crinkle" is the right (weave) and the one most straight will be up and down (warp).
If you are upset, do not work for long periods at a time.