There are a lot of superfluous makeup products out there; foundation primer, finishing powder, finishing spray. Eyeshadow primer, however, is not one of them. It really is something I would consider to be a necessity. There are quite a few products on the market- Too Faced's Shadow Insurance and Urban Decay's Primer Potion are the two most notorious. There are a few indie labels that have products along those lines as well. (See the bottom of this blog post for a list of helpful links.) Personally, I will swear by Urban Decay's Primer Potion. It's a daily makeup application essential. What's so magical about it, you ask? Did they bottle essence of unicorn? Maybe. What I've noticed it does for me is prolong eyeshadow color, make the color more vivid (even with less expensive drug store shadows), and help keep it in place. These are important factors to not only help you use less product, but to also help for those of us with naturally oily skin fight the battle of the crease line.
For those glamor addicts out there, I would also recommend Too Faced's Shadow Insurance Glitter Glue. It will literally glue the glitter to your face. It can also be mixed with loose glitter to make a nice glitter eyeliner. It is more difficult to wash off at the end of the night. You will have to scrub. But how else do you expect glitter to become glued to your face?
For those of you who are new to eyeshadow primers here is a helpful link list to get you started: