I got two medical bills recently, one for myself and one for my son's three year well check. Both were supposed to be covered, so I called the places. Turns out they were both errors in billing or insurance, and I wasn't responsible for either of them. Saved myself $30.24.
Last week I called to order a pizza. I ordered a large with toppings, and a small cheese. I asked if there were any deals for a free small, and the guy said "If you have our coupon, it's free with a large." So, I told him my dad was picking it up, so he wouldn't have a coupon, but asked if I could have the small for free anyway. He said yes!
The point is, it never hurts to ask- in fact, you'd be surprised what people are willing to do. And that's a very easy way to be frugal.