Washing up liquid really helps but make sure you condition your hair lots after- it gets rid of a lot of the colour, trust me ive done it before :)
My hair is naturally blonde and I dyed it to a dark brown (that had dark auburny undertones as a result of my natural blonde) a few months ago. Since then, I've box dyed it that same color just to keep it fresh looking and keep my roots at bay. Now I've decided to go back to blonde, and I've been asking everywhere for help. This is what I've gotten: Highlights are the best option if you are wanting to go back to blonde. You can do all over highlights repeatedly (every few weeks perhaps, but I'm not a professional, so don't quote me) until you reach your desired color. Bleaching it is VERY harmful to your hair and although you may reach the color you wanted, your hair will be damaged and gross for a long time. I agree that it's best to consult a professional about your specific type of hair and what is best for you.
Quite simply, you can't...do not try this yourself...you will be spending money on something or other that could have gone to a professional who will do it right the first time! This is nothing to mess with on your own~!