For me, visualizing myself encased in the energy of a specific protective crystal helps me shield myself effectively as spiritually I'm much more drawn to working with crystal healing. I change the "type" of shield based on what crystal I think is appropriate for the situation.
I think that's the trick, looking inwards at what you're drawn to and interested in and using that to help you visualize better. For some, they may be more interested in the violet light technique, others may find it easier to visualize a mirror's reflective surface. For me, it's crystals and their energy.
Shielding takes time to master, just like anything else which is something that isn't discussed or brought to the attention of beginners. I think it puts off many from working with energy when they are bombarded with different ways to shield and talk of how important it is, yet they can't seem to get it down the first few tries. So hang in there and keep at it :) You'll get it.