Overlay:
I've never tried a thin plastic film overlay, but I do have the Compaq hard-case thingy, which I find very useful (it's got a hard PVC shell over the screen on a hinge). Compaq part number 249703-B21
(can't find a URL right now 'cos my Internet access is kinda borked).
Basically it means I can sling my little iPAQ into my bag without fear that something is going to trash my screen, and it protects against dust pretty well too because it's almost flush to the device.
Screen-cleaner:
Same as MC - I use my glasses cleaning cloth and some screen-cleaner (but if you're getting it mucky you're probably doing something wrong

).
Cheers,
Ref.