Two words - don't overcook! Grass finished beef doesn't have the fat content to stand up to the higher internal temperatures that come with cooking to medium and above. What's more, it cooks 30% to 50% faster than what you're used to. Watch it closely! Rare and medium rare - that's what you want.
If you don't have a rapid-read meat thermometer, get one and use it regularly. Pull the meat off the grill or out of the oven when the internal temperature reaches 120-degrees, and let it rest, covered with a foil tent, for 10-20 minutes. It'll finish cooking while it rests. Generally speaking, if I'm working on the grill I'll touch the beef to see how firm it is. If it feels like my cheek it's rare... if it feels like the web between my thumb and forefinger it's medium rare.
We also suggest that you stick with thicker cuts. The steaks we sell at the farmer's market and through our CSA are cut no less than 1 1/4" thick, and we suggest that our custom-beef clients get them cut the same way. If you have a thinner steak than that, throw it on the grill still partially frozen. That'll help you keep the internal temperature down while the surfaces are getting a nice char going.
Another thing - because the beef has such a low fat content, you'll want to use oil on the grill every time you throw something on the barbecue. It'll stick otherwise. Some folks use tinfoil under their hamburgers. We like the char taste that comes with direct grilling, but experiment and see what works best for you. Ken likes grilling one side of his burgers on foil, for example, to make sure they stick together when he flips them. I have great luck without it. You make the call.
When you form your hamburgers - again, form them about 1" thick. I'll roll the burger into balls, flatten them to about 1" thickness, and then make a divot in the middle with my thumbs. When it cooks, the meat seems to rise up in the middle... if you put a divot there it'll rise up to be flat when it's done cooking. Whatever you do, don't make thin patties out of this burger. You'll be disappointed if you do.
If you like your beef cooked past medium-rare, we strongly recommend that you just give our meat a try cooked the way we recommend. You'll see. We are confident that we'll make a believer out of you.