What is the easiest way to create a background image that doesn't have noticeable edges when repeated?

For example, let's say I wanted to create a background of stars... how can I make a 100x100 image of stars that will look like one big sky when repeated, and not like a bunch of square tiles?

I use Photoshop, if that makes a difference.

Thanks! I'll need a bit of practice but it works great. Much easier than I expected.  

she said what i was going to say. good job  

Autarkis touched up on this a bit, but I'll go ahead and clarify.  If you do Filter->Render->Clouds and do it at a size that is divisible by 64, it will automatically do a seamless texture.  

When doing clouds, your foreground and background colours determine the makeup of the clouds, so go with something like black and dark blue for a nice night sky colour.

The stars can be done several different ways, depending on the type of stars you want.  Here is a method for a semi-realistic star field:

-Create a new layer above your black and blue cloud layer and fill it with black.
-Filter->Noise->Add Noise and select about 50% for the amount, gaussian for the distro and check off monochromatic.

-Filter->Blur->Gaussian Blur and do between 1 and 2 pixels.  This will soften the noise a bit and give us different sized stars later on.

-Image->Adjustments->Thresholds.  Here you'll see a histogram that looks something like an anthill.  At the bottom of the histogram, you'll see a little arrow that you can move.  Move the arrow towards the right side of the "ant hill" and you'll notice your stars will start to pop out.  The further over you move the arrow, the more the stars will pop out.  Having it about 3/4ths to the right seems about right for this.

-Filter->Blur->Gaussian Blur again at around 1 or 2 to soften the edges.  

-You should now have what look like stars on a black background.  Change your blending mode for the layer with the stars on it to Screen.  You should now have stars against a night sky.

-Now you can use Filter->Other->Offset on your star layer to make sure it's seamless.  For the number of pixels to shift, you should do half of your total image size for both horizontal and vertical.  If you are going with 64x64, horizontal should be 32 and vertical should be 32.

-Now that you've offset the stars, you'll see some large gaps where there aren't any stars and some "half stars" where there was a star drawn on the border.  You can make new stars with a light grey brush and delete half stars with a black brush.  

-Do Filter->Other->Offset a few more times just to make sure you fixed up any seams.

And that's it for a simple star pattern.  If you want to make a larger one with planets and nebulas (nebuli?  nebulae?  Whatever the word is for more than one nebula), then just follow the same steps with the offsetting, fixing seams, and offsetting again.

I'm going to do one up really quickly here to make sure I know what I'm talking about.  I'll post it below.

Very cool... I can tell I still have a lot to learn about Photoshop.

Kel