Tip #1 - Put concealer on eyelids before eyeshadow.
Do not put your eyeshadow straight onto your bare eyelid.
You want to first apply concealer before the eyeshadow. This is because the concealer will cover any redness/pigmentation/little veins on your eyelids. This is like if you put on blush on your bare skin it wouldn't show up much. But, if you applied it on top of foundation on your skin, it's going to show up better. So, by applying a concealer on your eyelid first before eyeshadow, your eyeshadow colours will pop more.
Tip #2 - Set that concealer with powder.
If you're thinking - oh, I kinda put some of my foundation over my eyelids, is that the same? Well, yes, pretty much. Concealer is ideal because it's got a higher coverage but that's fine if you don't have one and can only use foundation. Either way, just make sure you're doing this tip.
Which is setting the concealer with a powder.
Why do you set it?
If you didn't, then when you blend your eyeshadow on your eyelids, it's going to be like walking through the mud. A sticky, difficult, trudge.
If you do set your eyelids, however, then blending your eyeshadow ill feel more like gliding on ice - a much easier movement.
The result is having a seamlessly blended eyeshadow look compared to a harsh line.
Tip #3 - Pat your shimmer on with your fingertip instead of a brush.
You don't always have to do this, it honestly depends on the type of shimmer. But, more often than not, I have found that most shimmers apply better on the eyelid when I have used a patting motion with my fingertip compared to using a brush. You just get more 'bang for your buck' or, pigment.