What's the best method to tenderize a tough looking sirloin steak about an inch thick?

I got a Needle tenderizer. It's great for tough cuts of meat. I have used it on some wal-mart steaks and it helped a lot. For people on a budget it's well worth the investment. I use it on chuck roasts that are not well marbled and it works well with those.
http://www.amazon.com/gp/product/B001347JK6/ref=pd_lpo_sbs_dp_ss_1?pf_rd_p=1944687702&pf_rd_s=lpo-top-stripe-1&pf_rd_t=201&pf_rd_i=B0000DDUCM&pf_rd_m=ATVPDKIKX0DER&pf_rd_r=05N3Q5ADYJNR1AD98E9E

I got mine at an Academy Sports in their BBQ section. Now would be perfect time to catch one on sale.

Salt it well and let it sit for at least an hour before cooking. Do not pat it dry before cooking.

Salt it well and let it sit for at least an hour before cooking. Do not pat it dry before cooking.

Agreed on the slicing - also you could use a marinade that would help it along - papaya is a natural tenderizer, you could marinate it in papaya juice and some savories - say, soy sauce, garlic, some fresh herbs and pepper.

While sirloins aren't the same kind of tender as fillets or rib eyes, they aren't tough the way a brisket would be. As amysarah said, cutting it in slices across the grain is a nice way to handle a sirloin.

I've never found sirloin steak to be a tough cut. I'd just cook (broil, grill...) as usual, and maybe slice it rather thinly across the grain, if you're worried it's not tender.