This is one of those obvious tips that some of us have a harder time with than others, but it is one that can really make a difference in how much you spend each shopping trip:
Before going grocery shopping, look at the ads for your store(s) of choice and make a list of the sale items you need, plus any other items that really can't wait until they go on sale (which I find is usually very minimal). Then stick with your list!
Sometimes there are items that, while looking at the ads, I think "Well we don't really need that, but it sure looks good and it's on sale." If I want it enough, I'll add it to the list too, but in parentheses. Then the day I go shopping, I'll see those parentheses, do some math, and see how I feel. More often than not, I won't feel the urge to buy that item anymore and feel good instead about crossing one more thing off my list without adding to the allotted $35.