The one drawback to using lava rocks in your grill is that they can absorb dripped fat and debris, potentially causing the very flare-ups you are hoping to minimize through their use. I've found that a bit of maintenance, like keeping the heat on after you're done cooking, generally helps cook out any fat accumulated during use. 
Both bent metal sheets and perforated metal sheets share a certain set of issues when it comes to their use as heat diffusers, even though they have become the industry standards. The first is they’re not adjustable, so while they tend to mitigate uneven heat distribution, they really can't be made to compensate for it in the same way lava rocks can.  When it comes to flare-ups, the bent metal sheets, which generally just cover the flame pattern of your grill, allow a significant amount of fat to drip past the flames, reducing the potential for flare-ups. However, the fat that does drip on the sheets, both bent and perforated, either flares up or burns up, leaving a hard-to-clean and kind of stinky residue, another factor you should consider while cleaning your grill.
I've read that you should never replace these metal grills with lava rocks, but I've done it twice with great success. It does require the addition of a support structure for the lava rocks, which may not be easy to fabricate, but the results convince me that it’s worth the effort. 
No matter what type of heat diffuser you have in your grill, it needs to come out and be thoroughly scrubbed down. A soak in a little cleaning solution, and some serious elbow grease, may very well be required. But in keeping everything clean, you’re eliminating the potential for off flavors in your grilled foods. 
Step Three: Get the Jets Out
The last piece of the puzzle (for gas grillers, anyway) is the gas distribution system. This is where many of the issues with gas grills originate. If your gas distribution is uneven, there's little you can do to fix it, so take the time to get things right. Remove the gas distribution manifolds and clean them gently with a wire brush, making sure all the jets (the holes where gas comes out) are clean and obstruction-free. In some cases, it's worth soaking the manifolds in cleaning solution before brushing them, remembering to rinse and dry them thoroughly before reassembling your grill. 
Be very careful when disassembling and reassembling your gas distribution system. Even having a slight tilt to a manifold will significantly alter the distribution of gas and resulting flame pattern of your grill, and sadly the only true test of your reassembly skills happens once everything is put back together. So unless you want to become a master of grill disassembly and assembly, be slow and methodical here. 
You should now have an empty grill frame in front of you, a great opportunity to scrape down, brush out and hose down your grill to help keep it working properly for a long time. Make sure to get into the corners and remove any accumulation of debris that could cause headaches down the road. Once you're finished cleaning your grill frame, make sure to let it dry before beginning reassembly.
Step Four: Test Your Toast
It's the moment of truth. First you have to remember exactly how all your pieces go back together, and then you'll need to hope you did everything correctly. Here's an easy way to find out. Cover your grilling surface with slices of white bread. Turn the grill on high and let it heat up for a few minutes, then turn the heat off. You'll be able to see how evenly or unevenly your grill is heating by looking at the patterns on the toast. Hot spots will be burnt while cool spots might still be untested. It's not a perfect test, but as close to perfect as you're likely to ever need!
Assuming your toast-test went well, congratulations! You should be ready to grill now, or, if you're doing it right, ready to store your grill for next year. Most of you might be playing catch-up, but emember to clean your grill at the end of the season so that next year, you’re ready to fire it up just as soon as the grilling bug hits. And remember that regular maintenance will make your annual cleanup easier. Each time you've finished grilling, let the grill continue to run for a few minutes to burn off debris, and then brush down your grill with a grill brush. You never know when you might need an emergency grilling session, and it's better to put in a little extra cleanup each time than risk grilling up a sorry, crusty burger.