Size your artboard to the total size of the comic strip.
On a layer above your comics artwork, make picture frames (probably just rectangles with a small black stroke and a large white stroke set to Outer stroke alignment) that effectively "mask" everything below, in the sense that nothing outside each black stroke is visible. Keep this layer locked.
You can even set the picture frames layer's opacity to 50% while you work, so you don't lose objects underneath and can always find them easily, without having to hide/unhide the layer. Alternatively, you could CTRL left-click the layer thumbnail and turn it to outlines mode to keep a "preview" of where the picture frames are, even less obtrusively.
When done, just export a JPG or PNG, being sure to designate the artboard you want to save, at the bottom of the export dialogue window (even if you only have one artboard still do this - it tells Illy to ignore everything outside the artboard, otherwise you'll have random white, blank space in your exported image as far as all "masked" objects extend).
As Scott wrote Clipping mask
Draw Rectangle the size of the frame you need select the art and rectangle then command or control 7.
Could be simpler.
Or if you need it simpler you could draw yyour rectaangle first select it go to the Draw Inside tool near the bottom of the Tool Panel and draw away. It's a drop down in CS 6
Nothing beyond outside of the rectangle will be visible.
Draw Inside will allow you to reposition the rectangle or frame if you wish in either the relative position tothe art or displace the rectangle and the art as a unit.
If you select a path within the frames you will be able to see the entire path include what lies outside the frame for editing purposes.
Read about the Draw inside feture and you might also find the Dtaw behind feture helpful for your work as well.
Ahhh, Wade, that is AWESOME! EXACTLY what I've wanted. However, I am more accustomed to Adobe Fireworks. (I posted in this forum because I was more certain Illustrator has this feature.) Before I mark you as correct (which I think locks up the thread), do you know much about Fireworks? If so, is there a similar feature in that software? (I'd rather not switch software, but definitely would for this simple little feature.)
They hage a paste inside command and masking features but to tell you the truth they seem a bit odd to me definitely not as simple to understand as Illustrator.
You canread this article but I have to say everytime someone tells me how easy it is to understand one of the old macromedia applications I wonder wht they are thinking. It seems like nothing works like one would expect it.
here is the article masks in Fireworks.
BTW if you mark an answer answered it does not lock the thread and you can mark others as the helpful.
If you do not want to amrk something as the answer as you are not certain either mark it later or you might mark it helpful.
You caqn marrk other responses as helpful as well.
But make certai9n you have the right answer before marking any response as the answer.
Oh, is that so? Helpful answers to all it is!
By the way, I posted the same question in the fireworks forum and people gave me the answer you did. I think I'm going to switch to Illustrator after all. It has much more going for it, a sexier dark UI; just going to have to deal with the learning curve hah.