For the arcing superimposed lines, try to be a little less conscious of the guideline. It’s more important for them to be confident, than it is for them to be accurate. This goes for the ghosted lines, too. In the planes exercise, be careful that the non-diagonal center lines go through the center point of the plane (that is, the intersection of the diagonal center lines.) Finally, if a line stops short, don’t extend it- leave it. Correcting your mistakes just makes them stand out more.