Unless you’re modelling something made of only perfectly flat surfaces , you’re always going to run into non planar faces, even more if you’re modelling low poly characters for games that own animations can distort faces that were perfectly planar at first.
Usually when i had a game asset that had one or more non planar face that were deformed enough to not being displayed by a specific engine, i simply triangulated such quad, because triangles are always planar and so will fix a “non planar quad with too much distorted angle” problem easily.
To make it fast, first simply complete your model the best way you can without worrying too much about deformed quads.
Once done, enable the Mesh : 3D Print Toolbox addon that comes with Blender (File -> User Preferences -> Addon panel and in the Mesh category).
Select your model and go to Edit Mode, on the toolshelf click on the now present “Print3D” tab (blender 2.70) and click on the “Distorted” button
By default the angle it will check for non planarity is 45° but you can lower or increase it according to your need.
It will display then display the number of faces that it found to be non planar according to that angle
Just click on that button and it will automatically select those faces on your model.
From there a simple CTRL+F -> Triangulate will fix your problem.
And if triangulated face break the shading by having the new edge not in the correct orientation, just select the edge and press CTRL+E -> Rotate Edge