There are a few necessary tools you need to have in order to really complete a proper eyeshadow look.
You need a brush to pack it on, a brush to get it in the crease, a brush to blend and a brush to line. I'm not saying you can't find dupes of these particular brushes I'm about to recommend to you somewhere else, however, I have these brushes, tried to find cheap dupes and it was hard. I did find an ELF brush (cheap-o-la!) called "e.l.f. eye shadow brush" that was wonderful, in addition to these brushes.
1st - MAC #239 brush, which is soft, white and has a rounded top. It's very versatile. I do not use it with oily products, however. I use it strictly with powders (shadow, pigments). You can use it wet (please never lick it! Seriously, some people do this- it's very unsanitary!!!!!) This brush is great for placing shadow in the crease and outer top lid area.
2nd- MAC #242 which CAN be used with emollients and works great for putting on a base (Too Faced Shadow Insurance, or a MAC paint pot, or a MAC fluidline, which is what I love it for the most). It can also be used with powders. I LOVE this brush for picking up shadow and packing it on my upper lid. I don't use it for blending. You'd see why if you had it... it hurts to try and blend with this brush. You'll learn as you go, I promise.
3rd- MAC #217. It's white, dense and GREAT GREAT GREAT at blending in that crease and outer edges. I used to have trouble doing a crease. WOW! I used this, picked up some dark shadow and popped it in the outer corner and blended it for a second and it was perfect! It took me four years of owning this brush before I realized this!
4th- MAC #263 angle brush (LINER). Synthetic fibers, easy to clean. Great for corners. Works with all kinds of formulas... liquid, powder, oil based, etc. Makes a perfect line for me, especially making "winged" liner that you want close to the lashes (less noticeable).
For photos of these brushes please refer to MAC's website.
It is a small investment, but if you are serious about your shadow and want to make a noticeable difference, this is the way to go.
BTW, these brushes can be cleaned easily w/ a gentle cleanser. Many people like to use baby shampoo.