The key here is to have your students try out, or demo, several different frames before they make their purchase. Once they have narrowed their choices, selected the racquet that feels best to them and made their purchase, that's when the real work of customizing begins. Hopefully, your student has chosen a racquet that feels pretty good to them, so drastic changes should not be needed. If your student is resistant to changing the characteristics of their frame, you might remind them that even tour players, who are very careful to choose the very best racquet, still customize their equipment. The main factors to be aware of when customizing a player's frame are weight, swingweight, torsional stability and sweetspot.
Weight is fairly obvious.
It is how much the racquet weighs when you put it on a scale. Weight is important because it indicates how hard a racquet is to move forward. Remember, even short choppy strokes still travel forward. Swingweight is a little more complicated. This is important because it is a measure of how hard it is to get the head around the point of contact.