Also try cooking it to a higher temp. I cook mine to right around 150 and if sliced correctly and thinly, it's very tender, very moist and flavorful. Also resting in foil for 30-45 minutes seems to help.
I use Lindberg Snider Porterhouse and Roast Seasoning on my tri-tips and then smoke them at 250 degrees. Once it's done I brush with a higher end teriyaki sauce and allow it to rest in foil. Lastly I dredge the sliced meat through the juices which collected in the foil when serving.