aura shield, or perhaps more accurately a colour shield.
Center yourself, and then ground. Start by visualizing energy coming from your center to surround you. You are surrounding yourself in a field of energy that is both intense in colour, and light. It is as if you are creating a second aura around yourself. If you are aware of your aura, it should feel as if you are expanding and strengthening its intensity. You should feel, when you are creating the shield, as if you are "glowing" - you are projecting a very bright field of energy. Make it whatever colour pleases you most; I usually make it the colour of my aura, but a colour appropriate to your purpose is also a good idea.
While you are creating this field of energy, remember to visualize that negative energy is absorbed by the shield and sent down to the Earth to be grounded and cleansed. Also visualize positive energy coming to you through the shield. Positive energy should always be able to flow through your shields to you. Built the intensity as much as you desire in the shield, and then stop when you are comfortable with it. The shield will be there, even if you don't think about it at all later.
When you stop visualizing the shield creation and stop giving it energy from the Earth, it should maintain both colour and brightness. It will stay around you for as long as you wish it to - if you wish to get rid of it, just do the visualization in reverse and take it down. It's also a good idea to refresh the energy in it every couple of days, at least, if not every day. It won't need a lot of maintenance, but it's always good to keep it healthy.
After mastering this basic shielding technique, you may want to visualize yourself some custom shields. For example, some people like to visualize their shields as a constantly moving flow of water or fire - fire to burn away negative energy, or water to flow it down to the Earth Power again. If you don't like my techniques, play with them....you may end up making a shield that's completely unique and perfect for you