When couponing...do you have to buy only what's on sale + your coupons?

How do you know what to buy each week? I find that I save more money planning meals and sticking to that list, but not all my meals require coupons. I spent much more money just going to the store with coupons and grabbing stuff that was on sale with coupons. I have been to sites like southernsavers.com for a list of sales and matches to coupons, but then I get to the store and there's more stuff on sale that was not listed that looks good. Most of the time I walk out with tons of snacks and random condiments and cool new household cleaners, but nothing to actually make a meal out of (unless I am not just creative enough). Does anyone have any suggestions on how I can continue meal planning, while keeping up with the sales and the coupons, as well as manage to stock up on the side? I am shopping at three different stores each week. I am overwhelmed and accomplishing little. Help!