One of the few poorly-engineered areas of the Jeep Wrangler is its distinctive grille. It's not that the traditional 7-slat grille isn't attractive or functional, it's that the large openings allow leaves and stones to clog and damage the radiator's fins - there's nothing between the slats to prevent junk from getting in there.
While continuing to plan my big-ticket mods to the Jeep, I spent a free weekend evening performing this quick and easy fix: gluing some zip tie blocks to the backside of the grille, which secures some mesh across the grille slat openings. I didn't happen to have any expanded metal lying around, and my local hardware stores would only sell it in bulk, so instead I opted to use some inexpensive gutter-guard material.
I could have used aluminum gutter guard, but I wanted this to be a subtle mod, which would have required painting the silver metal black. The problem is that the paint is likely to chip off, even if I spent a good deal of time cleaning, primering, and then painting the mesh. Instead of going through that hassle, I opted to use a cheap roll of black plastic gutter guard material and used some heavy-duty scissors to cut 7 individual pieces which I oriented vertically behind each slat and affixed with zip ties. So far it looks great and is holding up just fine, but if the plastic winds up stretching or breaking down over time due to sunlight and ozone, I can always cut the zip ties and attach some aluminum gutter guard to the same zip tie blocks in place of the plastic gutter guard.
I didn't bother to post my own how-to because this is such a common mod, you can find pictures and discussion on just about every Jeep forum on the Internet. JKOwners.com is worth checking out.