I watercolored a portrait of my son and my daughter for a quick, last-minute Christmas gift. The truth is, though, that they're not very good paintings. They both have that awful "art student" look. It's hard to identify the specific problems, but I have two practical pieces of advice for myself:
A) Don't overwork. I toyed with the portrait above for too long, got too uptight, and tried to add too many layers and scrub away old layers to atone for my mistakes. Working faster gets fresher results but requires more accuracy. In sum: practice.
B) Get some contrast in there, dang it. A lot of amateur watercolors are too pale and anemic. It always dries lighter than it looks wet. Work with a higher paint-to-water ratio - more than you think you need - and perhaps pigment from the tube rather than from dry cakes will make that more natural. You can always add layers, but then you run the risk of problem A.