You do it like any other addiction, start limiting your intake and slowly reduce the amount your ingesting. Try switching to another product that doesn't have aspartame in it, although I think they switched the name of the stuff to something else {I heard on the news} because peeps were getting wise to it and not buying it.
For everyone who wants to keep up on how they slip aspartame into our foods, drugs, vaccines (yes, vaccines), OTC meds (especially child products), aspartame has a new name of AminoSweet. Since aspartame has gotten such a bad name (as it should), Ajinomoto has renamed this toxic chemical sweetener. Start double checking your labels.