Tip # 1 If you cook a squid head/body/thing whole, like a cutlet (or "squidlet"), the heat will make the meat curl, rendering it nearly impossible to cook evenly. This happened to me once with a Tilapia (aka "the hippo poo fish") and I could have used the thing as a boomerang after I grilled it. Horrible waste.
If you want to keep the squid from curling up on you, score it deeply with a knife diagonally in both directions. This is why squid in so many restaurants have that distinctive cross-hatch pattern on it when it shows up at your table.
Tip # 2 If you want to insure that your squid or octopus is tender, soak it in buttermilk overnight. Buttermilk will reduce strong tastes and help tenderize. It can also be used for game meats and, I've been told, works wonders in taking the "fishy" taste out of frog legs. Another way to tenderize cephalopods is to wack them against some rocks. No, I'm serious.
Tip # 3 If you're looking for cephalopods to eat, you'll find a large selection at T&T supermarket. They've got a great selection: heads only, tentacles only, octopus, squid, cuttlefish, whole, cleaned, cut, parts, dried, whatever you want.