For black and white versions or different aspect ratios, I'd create different "virtual copies" as suggested before. You just right click on the thumbnail and select "Create Virtual Copy" (or something like that). It won't create a new file, but will create a virtual copy within the Lightroom catalog with the settings you specify. You can even give it its own metadata, so for all intents and purposes, as long as you're within the Lightroom environment, you can treat it like a second copy of the original photo. From there, you just export each copy. You can of course select all of the copies at once and export them all in one fell swoop. You'll first need to set your export parameters, however.
Like 110yd, I'm not in front of my computer, but basically, you just right click on the thumbnail of the image and one of the options is "Export". There will be a bunch of presets, but since I'm guessing you haven't set up any of your own yet, I'd just go to the top one, which I'm pretty sure is just called "Export". It will bring up a dialog box with a ton of options. In here, you can decide how you want the exported image to be named, what size it should be (though even if it's possible, this probably wouldn't be a good place to change from an 8x12 to 8x10), file type, color space, etc. You can then save this preset so you can use it in the future without having to input all this data again.