i'm sure everyone has their own tips they have found out over the years.
I don't really have anything i can see as usefull myself but i'm sure most people will else does.
Lets see...
if i had to put one tip i'll say not use too many colours that are too similar. People's eyes aren't that good so they'll hardly notice.
Usually less colours look better anyway.
With pixel animation, try to appreciate that movement affects the whole body.
The more of the image you alter and redraw, the more squidgy and lifelike the sprite will appear. If the leg moves, chances are the back, shoulders and upper body may also need to move as well.
More of a GUI tip: Don't use red/green on numbers, indicators etc as the only way to know if things are good/bad. My flatmate is red/green color blind, wich is pretty common. He often has trouble playing games such as oblivion where red and green numbers indicate boosted/drained attributes on your character. Makes for some funny moments, but he really can't tell the difference.
Its hard to be religious when certain people are never incinerated by bolts of lightning.
Really basic thing I just learnt - for darkness don't always add transparent black pixels over the top of colour (if that's your style), when making the colour ramp try going to grey instead (of course different materials - different ramps). My art teacher said always try to avoid using black in art. Guess it might apply with pixel art too...
One thing I found is that it's much easier to animate single-color shapes than highly detailed characters. If you get a nice-looking rough animation first, you can refine it afterwards, adding shading, outlines, what-have-you.
Maybe not much of a tip, but it's one way of doing things.