Some eyes are just to pretty, but I bet some of you would change that if you could. I am going to try to explain how to make eyes look more sinister, And remember this is coming from a begginer,so bare with me. I used Photoshop CS2 for this tut.
Before and After
I will show you how to go from this

into this

Getting started

Well first off you need an eye to work on(get a big and high resolutioned one). Or you can grab this one.

First duplicate the eye layer(and work on the
duplicated one so you can toogle back and forth later, and see the changes),to do this go to your "layer/channels/paths" window,right click the word background and chose "duplicate layer". I marked the window with red.

The Darkening

Then select the burn tool and set it's range to midtones
and exposer to 35%(the burn tool is labeled 1 and its options are circled),

then burn all around the edges of the iris(the start of the eye, the colored part not the white part) till it's almost black but not quite black,then increase the size of the brush and burn the
pupil(the black part of the eye) it should resemble this now.

Return of the Darkening

Then use the burn tool around the
whole eye(right on the eyelash line) and apply it like eyeshadow till
it starts to look heavy and dark,
like this,

Then grab the dodge tool set its range to midtones and its exposer to 35%(this is the same place the burn tool is located,just right click the burn tool icon and click on the dodge tool or you can hold down the ALT key while you burn and instead of burning it will dodge) and go inside/inbetween of the iris and pupil and make it glow.

The color layer

Now create a new layer(to do this go to your "layers/channels/paths" window and click on the icon I circled and set the blending mode to color(to do this you make sure that the new layer is active or highlighted blue, and at the top of the layers channel you'll see a box with an arrow pointing down and inside
the box it will read "normal" click on it and a list of options will come up, change "normal" to "color"(is it all the way towards the bottom) click it, and reduce the opacity(it is located at the right of the box you just changed) of that layer to 50%, now get a nice bright

color(on your tools just double click on the top color box),now click the paintbrush tool make the mode of brush normal and set the opacity of it(the brush) to 100 and the flow to 100 and choose a brush size so that it fits inbetweeen the iris and the pupil(and keep all the sizes of the brush to a rather soft edge)and paint inside of it(I chose an orange/yellowish color).

The Glow

Now click on to your "background copy" layer again and select your dodge tool again and go right around the edge of the pupil(with a very small brush) a few times.
To get this, Then increase the brush size and go all around and inbetween the iris and pupil where you colored.

Pac Attack

With the "background copy" layer still active, go to
"filter-render-lighting effects" and click it,you'll see a diagram of your eye,now make the light come from the top of your eye(this is the only way I can explain it,you'll see an oval with four little squares and it will have a mouth like pacman)(LOL),the mouth of pacman is the focal point of light,put the mouth/slit above the top of the eye or any direction you think it looks best,now for the rest of the diagram,(lots of stuff huh?) well don't let it all scare ya(we will be out of this window in a jiff), you'll see two boxes to the right, pick the top one and change the color to a dark/pale looking color(I chose a dull orange)and hit ok.

This is the reaction Now your eye should look pretty creepy about now, but if it looks to colorful you can try this: with your background copy layer still active go to "create new fill or adjustment layer" I circled it here

(that is located right under
your background and layers pallet and looks like a "ying yang" sort of, it is a circle that is halved and is black on one side and white on the other), click it and go to hue/saturation and click that,now go to the saturate and decrease it till you have a more likable product and hit
ok.
You should have something like this now.

Decolorfie the eye

Now go to your color layer and make it active(highlighted blue),with it active hover your hand/arrow over the square(the little picture of your color layer) and hold the control button down and click it(you'll see in the main picture(the big one)that the color you have painted in is now selected with the little ants(the little moving things that make you know that you have selected something), now as before click on "create new fill or adjustment layer" then click on the brightness/contrast one,increase the contrast some and decrease the brightness some till you get something you like and click ok. I kinda liked this option. :)

Now if you don't like the color you can change it,just hover your hand/arrow over the color layer you made and control click it again(you will see the ants dancing again on the main picture),and go to the "create fill or adjustment layer" then go to hue/saturation and use your hue slider bar till you get your prefered color and click ok. And there you have it an evil eye! Sorry for all the long drawn out explaining but it may be necessary for some people(I know I could have used it in some of the tutorials I've tried). This tutorial is aimed at the begginer(like myself) so they can more fully comprehend and understand. (P.S. if you have any questions just ask and I will try to explain it some more).