I used to buy things and leave them in my closet with the tags on them for ages. Nowadays I go through my closet every month and anything with the tags still attached goes right back where I got it from.
I no longer impulse buy. If I am at Target or another store like that, I make a mental note of something I've seen that I think I might want to buy. Then if I remember it later and it plays on my mind, I may get it. Or I may not. Mostly I spend money (and clip coupons too) on the essentials...food, toiletries like deodorant, toothpaste, shampoo, etc. There is a big difference between needing and wanting and a lot of people forgot that while they used their houses as ATMs.
Instead of renting a movie I go through my pretty big collection of DVDs to find something to watch, or I find something on TV already, since I am already paying for it. Might as well right?
I found that since I always fell asleep at the movies I would save a ton by napping at home for free. So that is what I do now!
I do like to drink diet sodas so instead of buying them when I've run out, I stock up when they're on sale and store them in the garage. I never run out. Same with cat litter and toilet paper. It will all get used eventually so if it is on sale and I have a good coupon, I stock up!
If I have a smaller painting project in the house, I hit the "oops" table at Home Depot and sometimes find something suitable for a lot less than what a custom-mixed can would cost.
I hang pants and flat-dry sweaters to make them last longer (they fade otherwise), and also do not use fabric softener (makes fabrics wear out faster). I have a sewing kit so I can fix small rips and sew buttons back on instead of buying a new pair of pants or a new sweater.
I use old ripped/stained towels or t-shirts to wash my car instead of buying new shammies for it. I also wash my car using a single bucket of water and park it so that when I rinse it off, the lawn gets watered too. I do not see the point of paying someone to wash my car. I also clean out my car's interior myself.
I make my own household cleaners; they are cheaper and usually work better anyway.
I clip coupons and check weekly specials to save as much as possible on groceries. I plan a shopping trip to make the best use of time/gas. I also buy generics for most things. Sadly my cats turn their finicky noses up at generic cat food!
I buy clothes with classic cuts/lines and in classic colors to make them last year after year. I recently gave away 2 old winter coats, but I had owned one of them for 10 years and the other for 8. I gave them away because they no longer fit me (lost 118 pounds). Both were in excellent condition. I plan to keep my new coat for at least 8 years and since it is a classic color/cut it will look good for those 8 years! Plus it is an incentive to maintain the weight loss. I also hit my mother's closet from time to time and borrow clothes or shoes.

Or costume jewelry!
No Starbucks, no McDonald's, none of it. I rarely ever eat fast food and I never do it spontaneously. Luckily in my new neighborhood there is not a single Starbucks but I was never a Starbucks addict anyway. Daily or weekly trips to Starbucks or for fast food lunches/dinners can really add up.
I pack a lunch to work most days. The only time I do not is if a team lunch is planned and those happen maybe once every 4-6 weeks.
Garage sales! One man's trash...
Machine-washable, durable mop heads and microfiber cleaning cloths instead of Swiffer pads and paper towels. I keep a little plastic basket in my laundry room for the dirties and wash them when the basket fills up.
Replacing incandescent bulbs with CFLs to save on electricity...and turning the lights off when I'm not in the room! The exterior floods are also being replaced with CFLs, gradually. CFL floodlights have been hard to find around here.
Growing my own herbs; I can buy a small perennial herb plant for what it would cost me to buy those herbs fresh at the supermarket. And now I have as much sage, thyme, mint, chives, rosemary, etc. as I can hope to use and then some. I let my neighbors snip from my huge rosemary shrub whenever they want. Why not?
I love these kinds of threads and I love reading other peoples' tips!