I get really tired of hearing how people hate sirloin because it's "too tough." Now, I know there's a whole lot of you out there saying right now "It IS tough, you moron!" Well, I'm here to tell that you're only slightly correct. Sirloin, especially top sirloin, can be very easily prepared to be incredibly tender, juicy and satisfying.
The main reason sirloin seems tough after cooking is that it's not marbled well like more expensive cuts of beef. More fat generally means more tender. This is because the muscle tissue within the cut breaks down more evenly and efficiently because of the fat infused into that muscle tissue. In other words, you don't have to know what you're doing to end up with a tender, juicy steak.
Sirloin is lean. Very lean. In fact, top sirloin has less than one fourth the fat contained in a rib-eye cut of the same size. That means you have to think before you cook:
Allow your steaks to warm up.
Meat placed on the grill at room temperature will cook much more evenly. Cooking unevenly results in the outside of the cut being tender and the inside being tough because the tissue never actually had time to break down.
Rub your steaks in a light oil.
Adding a thin layer of olive oil before you add your favorite spices will help seal in the meat's juices, without adding any heavy flavor to your steak. AND, before you email me and say "Alton Brown says that you can't lock in juices by searing meat" you might want to re-read the previous sentence. I said "help seal in the meat's juices." You can't make any cut of meat juicier by attempting to trap the moisture, but you can slow down the flow of juices into the grill from the bottom of the cut with simple science: oil and water don't mix.
Sear the meat.
Searing your steak on both sides instead of cooking one side and then the other will further prevent uneven cooking. It will also add flavor and help contain the cut's natural moisture in the middle while it's cooking. These juices will permeate the rest of the cut if you let it stand for a bit after you pull it from the grill.
Finish cooking indirectly.
Direct heat is the kiss of death to a lean cut of meat. Always leave open areas in your grill when building a fire or stacking your coals. This gives you a way to get the meat out of harms way but still keep them on the grill. Once the outside is seared on both sides, move your steaks to the side of the grill with no fire and close the lid. If you cook with gas, turn off one of the burners. If you can't do that, I'm sorry to inform you that you had a severe lapse in judgment when you purchased your grill. To finish, cook the meat slowly, checking frequently so you don't turn it into jerky. Grin like and idiot. You did it.
Now you know the secret. Stop making excuses and start cooking!