I wouldn't say it exists a best method. If someone cannot afford doing that, he/she will never place a Standing order. Actually, a Standing Order is not compulsory. But, you may tell them the advantages, namely having a Standing order will escape them from the worry of forgetting purchasing that object every month. Also, if that Standing Order assures their VPs for qualification, a Standing Order escapes them from the worry of forgetting to take actions for qualification.
