I've had experience with all of the below items, and it's a popular topic on coupon blogs - stuff you should NEVER pay retail for.
If you're new to couponing, start here.
At the Grocery Store:
--toothpaste, toothbrushes (and floss) - everyone knows a dentist visit will give you some free sample to use, but full size products are always going on sale too. Sales for these items typically go down to .99, and this is easily combined with 2 for $2 and $1 off coupons. These are items that should always be FREE for you!
--Deodorant - secret and old spice always have $1-$2 coupons, which are quickly combined with store sales for those amounts. Again, something that should always be FREE
--Candles - wait for the clearance sales and combine with the glad candle coupons that are always around for free or nearly free candles.
--Air fresheners - Glade and Febreeze usually have coupons around, and I usually get these brands for free when I wait for a good sale to double up on a deal.
--Dish soap - Palmolive and dawn have coupons, and a couple times a year, there are great sales that make high volume liquids for very cheap. On top of this, many dishwasher companies will have "full value" mail in rebates zoo try their products. I don't even have a dishwasher, and I have a few dishwasher cleaning products (would last me a few months when undo get a dishwasher haha)
--Paper products - no brainer if you've ever walked that aisle! Toilet paper, Kleenex and paper towel are always on sale! Combine great sales with coupons for that brand which are always around and never pay more than a toonie for 24 rolls! I even got expensive hot pink toilet paper for $1 because there was an in store sale and a promo coupon from the brand.
-- food items: Soup, cheese, bread, Pasta (for the most part), bottled water. - these are items there are always coupons for and/or on sale. We've never paid more than $1 for a case of water. Cheese is expensive, so wait for the half off sales and combine with a coupon then. Same with pasta and sauce, it went on sale last week for $1 for a huge bag or one can.
--Laundry soap and softener - same deal as above. Our favourite has been the arm and hammer massive jugs that go on sale for $12, combined with $3 off coupons. It's 5 months and were still on one jug!
--Cleaning supplies - windex and green works have been my best scores so far, as free on buy 2 get $4 deals when it went on sale for $1.99 each.
The key for all of these is hanging onto your "high value" coupons until a good sale comes along for them. In some cases though, check those app rebates which may add onto a current sale + coupon to make the item completely free - I would use my coupon in these instances too.
Not in the grocery store:
--Interest on credit cards - why? If you can't afford to pay for it, don't put it on the credit card! My favourite line on my credit card bill is the line about how long it would take to pay off my bill if I only made the minimum payment. Never! Credit cards have crazy interest rates, and this is how they make money.
--Atm fees - plan ahead or shop at a place that gives cash back. Even buying a pack of gum and getting cash back at Walmart can be cheaper than using their $2 ATM fee across from the registers.
--Calendars - if you need one, these are easily printed or requested like the Milk calendar.
--Return address labels - this is probably a tasteless one, but many charities will send out packages with your name and mailing address on them in the hopes that you will make a donation. War Amps, SPCA and others.
--Grocery bags - buy some ikea ones once and always leave them in your car. 5 cents a bag adds up quick, plus you're helping the environment not contributing. Some places like NoFrills will have boxes at the front for you to put groceries in lieu of bags. If you absolutely need plastic bags, try shopping at Walmart where they are still free.
--Stuff you won't use - this seems simple but if there's an incredible sale on mustard but no one eats it in your house, the money spent would be a waste UNLESS you donate the item. I've done this with diapers and wipes when the deal was too amazing to pass up, but I don't have a baby.