I used to shop ad hoc, and there's no doubt it massively increased my costs and my wastage.
I swapped, at first, to shopping every 5 days or so. We had a relatively small fridge which was the first barrier. And the other was that I wasn't used to the level of planning required to make it work. We recently got a second fridge and I've got used to planning so now I regularly do shop for a full week at a time, with very very minor additional shopping on top.
Co-Op, which is our local convenience store, has definitely lost a big chunk of their income as a result! 
and we are saving. There are a lot of things that are notably more expensive when purchasing from these sorts of shop and it very quickly adds up.
To give you an example - I meal plan, and buy all veggies for that week based on the mealplan. I also consider whether, if I buy one packet of vegetable A, I can use that vegetable twice to ensure I don't landup with waste. Ditto, if I know I'll have leftovers because I'm cooking the famous MN roast chicken, what do I plan to eat the following day to use up that chicken.
I also save by taking advantage of savings resulting from a) buying from bigger supermarkets and b) because I can store things so can buy the bigger tub or the BOGOHF offers etc. This is particularly true across pretty much every type of product from cleaning stuff, to meat, to alcohol.
A third benefit is that some of the "treat' foods get eaten within 2 days, no matter how much I buy or how often. So this way, the family know - I will buy x or y, but they only come once a week. After that, if you've eaten them all, tough. Another saving.