When I was putting together my sequence I found that it looked flat, and perhaps not very aesthetically pleasing. The colours were all bright and in your face, the tonal range across the screen was very flat and more like a graphic than a moving image sequence.
Here is an example:
I decided to solve this issue by introducing some Overlay effects. Layers in the sequence which effected everything below it to adjust how it looked. I used highly feathered masks to create a vignette effect around the edge of the screen which gave a great variation to the flat colours that existed before as you get further from the edges of the screen. I then created an inverted version of that mask to form a ellipse in the centre of the screen and instead of black use white to create a spotlight effect. I lowered the opacity of these layers to a low level so you could clearly see everything on the screen but enough to created a shaded effect.
I also wanted the areas in the Vignette to be less saturated than the rest so I used the same mask but changed the blend mode to Saturation so that it would turn it black and white, then adjusted the opacity to get the desired amount of de-saturation. This stops the colours on the edges from being too vibrant and draws focus to the centre.
Here is the same part of the sequence but with the added effects:
I believe these effects have really made a difference in how my piece looks. To save space on the timeline I put these effects in a pre-comp and this sits on top of all the other layers in my main composition.