I have discovered that the best way to avoid contamination is to get a graduated cylinder small enough to accurately measure small amounts of chemistry. I have a couple that measure 10ml in (1cc?) increments.

if you want to use pipettes, the best procedure is to have several on hand and never use the same one twice when measuring chemicals. This way you are sure you have a clean one for each different solution. Wash them up when you are done and you are ready to go next time.