One of the easiest ways to overspend is the upsell; an upsell is simply buying one or more things after you've already spent money.
A well-constructed sales pitch will always look like a natural enhancement to a purchase; and, usually their whole purpose is not to add value but to diminish your wallet.
Not all upsells are negative; but, being wary of them will often save money the gross majority of the time.
By way of example, here are several negative upsells:
Gas Station Car Wash
* Plan ahead: Rarely a good deal unless you're already planning on car wash
* Know their primary focus: Gas station car washes are not typically high-quality
Electronics Store Extended Warranty
* Plan ahead: You should already have some idea of how long it will actually last
* Looking forward: Instead, "spend" the money as savings for the next gadget
Online shipping
* Plan ahead: Purchase early enough that no rush (usually the worst value) is required
A far more effective use of resources would be to be prepared for the upsell; usually saying no is the best course of action.
References provided here: http://extramargin.blogspot.com/p/references.html
