Here is an outfit with the shirt loose. Not terrible, but not great either!
And with a quick tuck into the front of the pants... Better! I can see a waist! The legs look way longer. Woot! Woot!
Blousing the tuck out a bit.
This is the result. It looks a little less deliberate.
Here is the another tuck before. Again, not terrible, but the top is making me look larger than I am and my legs are looking short again.
And tucked! I look longer and leaner.
So basically the rules are:
-Do a partial tuck with long, loose blouses.
-Tuck in the front, where the fly of the pant (or skirt) would be.
-Tug on shirt until you are happy with the result. This is less a rule and more of a "trial and error" sort of thing. My Type A personality hates this. It does work, though. You can tell when the tuck is "right".
-Blouse the top out a bit if you are going for a more effortless look.