Do this for five or six repetitions each time you meditate. Afterwards, let the image of the shell fade, but know that it is there. With practice, you can mentally double the shield at will. Your psyche will double the shield temporarily. If you need more security in a situation, imagine a silver reflective coating on the shield which deflects incoming energies.
Shielding is simple but effective.
Practice at least twice daily. It is better to do regular, short practices than infrequent longer ones. As an added boost, image a power sign, religious symbol, or other token of overwhelming potency on your forehead and solar plexus when you meditate on the shield. Popular symbols include the Thor's Hammer, Odin Wheel, Cross, Star of David, Pentahram and Lunar Crescent. To aim magickal energy at a doorway, object, or in the direction of trouble, image blue-white light pouring from your fingertips.