The Pokémon Peninsula > Sprite Editing Tutorial
This tutorial basically offers some tips and tricks for editing and recolouring Pokémon sprites. If you need sprites to edit, check out this page.
Zoom in to about 800%. This zoom level is available in all graphics programs. It's a good level for sprites because of their size - 800% is large enough to see all the individual pixels, but still small enough to get a good idea of what the whole thing will look like.
Use the pencil tool set at 1px, and turn off all anti-aliasing - at such a small size, it just looks stupid.
If the original Pokémon was in bright colours, use bright colours to recolour; the same goes for muted or pastel colours.
When recolouring to black, leave some colour (eg. the cheeks on a Politoed) instead of making those parts grey or white - otherwise it just looks like you converted to greyscale.
Don't use too many different colours in a sprite. Use different shades of the same colour instead.
For each shade or colour in the original sprite, use one shade or colour in your recolour, and keep them from light to dark in the same order so the 3D shadow effect works properly.
When combining two sprites, pick Pokémon that are about the same size - there's nothing stranger than seeing Ho-oh's large wings on a tiny Pichu.
Try and suit the two Pokémon to each other - a winged Ponyta would look cool, but a winged Onix would just look really odd.