The whole tri-tips are a favorite in my household, and I love cooking them because they're quite easy to prepare.
When I'm rushed for time, I throw a couple under the broiler (10-15 min per side, for med. rare) with Montreal Steak seasoning. After it rests, slice fairly thin and serve with horseradish...
I found a FANTASTIC marinade/glaze recipe from Sara Moulton on Food TV's website: http://snipurl.com/ysuz
, which I use for special occasions, or even when we go camping. It has a lot of ingredients, and is a bit labor-intensive, but worth every minute spent! It also calls for flank steak in the recipe, but I've only used it on Tri-Tip.
A couple of pieces of advice: make more marinade than you think you'll need (I usually double the recipe) because you have to reduce it and glaze the steak as it's cooking. I always wait to glaze until the last 10 minutes or so, so that the glaze doesn't burn (due to all of the honey and juices in it) It's nice to have some extra glaze set aside, because you can serve it at the table if folks want more on their steak.
Hope you like it!