I consider myself to be a bit of a tri-tip master
3 really important things....
1. Rub (don't inject, don't marinade)- keep it simple. The beefy flavor of tri-tip can stand alone. Salt (be generous), Pepper, Garlic, Onion. Parsley flakes optional.
2. DON'T OVERCOOK IT!!! Most people say pull at 130-135, I say NO!! Pull at 125, and let it rest for 15-20 minutes. You are guaranteed tender steak this way. At 130-135, it may end up rising to 140, 145, and unless it's a really high quality piece, you've lost the magic of this cut.
3. Slice ACROSS THE GRAIN!!!! I can't tell you how many times I've had tri-tip sliced with the grain - CRUCIAL MISTAKE. For the most tender perfect slices, slice across the grain. The grain changes directions in the middle. If you think of the shape like a boomerang, you can cut pretty much down the center of it, then rotate each piece length wise.