Guidelines For Pairing Image Sprites

I’ve been working on a homepage in reducing the amount of background image requests by using sprites on it and wanted to solicit some general work patterns here.

What kind of images do you combine for your sprites -are they thematic or are they based on proximity of each other within the page?

Although I do see the upside of reducing the amount of images, are there guidelines anyone uses for size ceiling?

I only use them for small bits and pieces—such as rollover items, icons, menu background—nothing large like page backgrounds. The ideal is that the Sprite image not be too big a download; but beyond that, there’s no real limit to how many images you can include. Google uses an image with possibly hundreds of icons on it. Personally, I only use them for images than need to change instantly on hover.