This little trick works well in traditional rib smokers but especially well in charcoal grills since charcoal grills lack the firebox that cuts the spare ribs off from the heat. Since the meat is so much closer to the fire when using charcoal grills, this is a bit more important. What you want to do first is to create a water pouch that will allow you to keep the smoking pork ribs from getting scorched while at the same time providing moisture to the rib smoking chamber.
Start with 4 large sheets of heavy duty aluminum foil. It is better to have too much foil than too little because you can always fold over the excess or cut it off. Put two of the sheets together and crease the edges all the way around the perimeter. Do the same to the other two sheets. Now, you should be left with 2 double ply pieces of aluminum foil. Stack them together and roll 3 of the sides up about 1". Use a very tight roll because you don't want the water to escape. Basically, you are trying to make a "pocket" here, leaving one side open so that it can be filled with water. Ideally, you should try to make the water pocket about the same size as the width of the inside of whatever grill you are using. This water pocket barrier will be the divide between the smoking ribs and the fire. It does not have to be perfect.
Finally, fill it with water and seal up the final side. Try to keep the water packet flat, like a wall. Don't worry if it leaks a bit -- not a bid deal since that will only add more moisture to the smoking chamber. You can even do what I do - tear a small opening in the top corner of the pouch, which allows steam to escape and constantly baste your smoking ribs, while at the same time it allows for you to refill the water pouch when needed. Now, you should try to situate the water barrier in your charcoal grill (before starting the fire!) so that the fire will be on one side and the meat on the other. If you use a normal ribs smoker, then place the water pouch on the rib side of the smoker. Since most ribs smoker lids are rounded like barrels then it is a bit difficult to cut off all of the heat as some will no doubt escape over the top of this water barrier. But, it still can cut the overall heat down significantly, even for regular smoker grills. You can attach it to the sides of the grill to get it to stand upright or you can prop it up with a couple opened beer cans or bricks. Baste the smoked ribs throughout the process as well.
During the rib smoking process, be sure to check that the pouch is full of water. I have tried to add spices to the water but have never noticed the difference in taste. Nonetheless, this little water pouch trick is pretty easy and cheap to do and also has the added benefit of keeping your smoked ribs nice and tender because they won't be fried by furious exposure to the heat. Oh yes, the added moisture is just another little extra.