Vitis

For tougher/cheaper cuts you need an acid and a salt to break down the sinew. Some things you can use or use a mixture of:

Balsamic
Citrus lemon/lime
Soy
Hoisin
Italian Dressing

I like a mix of soy, balsamic, lime, brown sugar, ginger and green onions. Works great with flank steak.

My other trick I use on most steaks, from the toughest all the way up to tenderloin. I fill a spray bottle with balsamic. I spritz the steaks while they're on the grill and they get a nice crust on them from the carmelizing sugar. Yumm

~Vitis