Often you think you are being extra frugal, but it turns out what you're doing isn't really saving money in the long run. Apartment Therapy gives a run down of places where you may be costing yourself money.
Clipping coupons may seem like a great deal, but often you will buy things you wouldn't otherwise have just because you have a coupon for it. Also, coupons are usually for name brand items which often have a store brand version that is much cheaper than even the price of the name brand after coupons.
Another mistake people often make is always grabbing the cheapest version of something. This can be a mistake because the cheaper version may not last as long and need to be replaced sooner. Think carefully about how much the item is going to cost per use or wear and if it will make sense in the long run to go with the more expensive option.
Do It Yourself isn't always the cheapest option. Think about what materials you will need, as well as what the cost of your time will be, and you may decide that buying an already completed version makes more sense.
Another big thing that may seem cheaper is spending time to save money. But if you drive all over town to find the best price, was your time really worth a little bit of savings? Think about whether the money you save is worth the time and effort you are investing.