Instead of trying to talk yourself out of an impulse purchase because you are trying to save money, avoid that situation completely (you will probably lose anyway!) Do not take chances on falling short of your goal; Put the designated amount of money in your savings account at the beginning of the month, and you are done. Goal achieved.
Whatever you have left is the money you pay bills with, eat with, shop and play with. Stretch it however you can, knowing that you have already met your savings goal for the month.