With MSG. Just kidding (well, not really - I love MSG).

It varies so widely even at the same place, so I take a taste of the broth first to see what would work best. I always add lime, basil, a slice of hot pepper and sawtooth herb (I love it when this is available!). Sometimes I'll also add some of the available sauces, depending on the flavor profile.

If at home and the broth is bland, I'll add MSG and/or fish sauce. The last batch I made needed almost no doctoring - I used this recipe and it was awesome:

http://www.seriouseats.com/2012/09/t...amese-pho.html

There's an embedded link in this post that explains how the various bones and cuts of beef affect the end product in a pretty dramatic way.