Dear Noel Axle is right, layers in Photoshop is the best way and you can do this even in PS Elements (the basic version that's bundled with many printers and scanners). To simulate multiple exposures open all the required digital images, then choose one as your base image. Click on the first one you want to merge, then use Select>All, Ctrl+C (to copy the image), go back to the base image and use Ctrl+V to paste the file you copied. It will appear on its own separate layer, and you can then edit it as you wish - using the Eraser tool will reveal the base image below. Repeat for as many images as you want to merge, you can change the layer order by dragging them up and down the Layers palette.
However, the best way of merging images in this way is to change the blend mode of each layer - you do this on the Layers palette. This works particularly well with cave shots as there tend to be large dark areas, and changing the blend mode to 'Lighten' will reveal just what you want. When you've finished you can go to Layer>Flatten Image to compress all the layers, or save as a TIFF with all the layers intact, at the expense of a bigger file size.
Try it and see, or Google 'Photoshop Blend Modes', there are countless tutorials out there. This image was done with this technique, using four separate shots of Biffa in Ogof Ffynnon Ddu II's Big Chamber Near the Entrance. I stacked the layers, changed the blend mode to 'Lighten' then tweaked it slightly with the Eraser tool. I have a slightly better version that I haven't finished playing with that I'll post when I've done it.
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