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  taken from http://venosoft.cjb.net/
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I can't believe I forgot to do that, heh...I've done them so many times that
it's just something I take for granted now I guess.  I forgot what a pain in 
the butt it is to get used to doing tessellations.  I don't know when I'll be 
able to do a tutorial on it specifically (a lot of people are asking for 
animation stuff now, heh), but here are a few tips to get you started:

If you're using a weird tile size (like 17x17) you're going to have a lot of 
troubles because it doesn't divide nicely.  16x16 is generally used because 
it divides up into 4 pixel sections nicely.  So if you want to make a floor 
that repeats nicely, make the pieces of it (like the wooden planks or the 
rows of bricks) 4 pixels wide or tall.  If you were doing a brick wall or 
floor, you'd want bricks that are 3 pixels long with one pixel of space (for 
the cement or whatever they use in brick walls), so that you could repeat 4 
of them easily.  For vertical wooden planks or wooden walls, you'd want to 
make them 4 pixels wide as well...or 8 pixels and just have 2 to a tile, or 
2 pixels and have 8 to a tile.

If you're thinking "But what about ground that's made of rocks or something 
and isn't perfectly even all over?", that's a little trickier.  The main 
thing is to practice, but you have to be able to think about how your tile 
is going to look when it's sitting beside itself.  Like when you're working 
on the edge of a tile, imagine how it's going to look beside the other 
edge...and the corners are another part to watch because you can get large 
blobs of pixels from 4 corners touching each other.  A good thing to do is 
when you get the general look of the tile figured out (like where the rocks 
and such will go), cut and paste the tile beside itself, under itself, and 
then diagonal down from itself so you have a square of 4 tiles.  If the 
pixels blob together in places, then you look on the original tile and 
figure out where to fix it.

It's tricky to do at first, but after some practice it'll get a little 
easier.  Good luck with it!

- Tsugumo