Cook the Tri-Tip like a good steak. I like to do it on direct heat on my Weber Grill. About 10 to 12 minutes per side will do the trick. Approx. 140* to 145* internal will be great. I especially like Steve Raichlen's recipe. He uses a rub consisting of:
2 tsp coarse salt (kosher or sea)
2 tsp freshly ground black pepper
2 tsp garlic powder
2 tsp dried rosemary, crumbled between you fingers
1 sp dried oregano
Rub the tri-tip down well with the ingredients and let it set while getting the grill started. Do the 10 to 12 minute per side thing and you'll really enjoy the result. And by the way, this recipe can be found on page 184 of Steve Raichlen's BBQ USA 425 Fiery Recipes from all across America. It's a really good book of recipes and I recommend it. Also, when you pull the entree from the grill, let it set for about five minutes and slice perpendicular to the grain on a 45* slant.