Building a stockpile really happens when you find a good sale on specific items you use often and you have coupons to match up with the sale.
Mostly, building a stockpile is driven by the sales at stores in your area. So watch for really good prices on those items and match them with coupons for good stockpile prices.
Ketchup and sunscreen should be something you'll find good prices on because of the season we're in (grilling burgers and swimming ).
If you have Rite Aid, CVS or Walgreens, watch their ads for both those items as well as ketchup at all your area grocery stores.
Very soon you'll see the drug stores and office supply stores start running great prices on school type supplies, so be ready to snag those for your stockpile needs as well - this is the time of year I stockpile pens, paper clips and printer paper very cheap.
Just watch the sales and you'll be able to stockpile what you need, and not just ketchup and sunscreen