With a dry piece of meat and a clean grill it's time to preheat. This is very important. You want to turn all of your grill's main burners to high (I'll get to charcoal in a minute). Let the grill heat up for at least 10 minutes or until it reaches its maximum temperature (read you grills manual to find out the best preheating times). Your experience with this piece of equipment is very important, so use your common sense and knowledge of the grill to make sure it is as hot as you can get it.
Now you are ready to sear. You need everything in arms reach and you need to move quickly. Your grill has stored up about as much heat as it can and you want to keep it there. Lift the lid and get the meat onto your grill as quickly (and safely) as you can. If you know where the hot and cold spots are, aim for the hot parts. With the meat on the grill close the lid. This isn't as much to get the meat to cook all around as it is to keep the heat in the grill. Most experts will tell you that you are going to flip in one minute. That may or may not be the case. Again, your experience should guide you, but keeping a close eye on the clock is good if for no other reason than future reference. Like I said you want to get a good, dark brown color on the meat, but you don't want to be lifting the lid and the meat repeatedly to see when you get that.
Once the meat is properly browned you need to flip it to a previously unused part of your cooking grate. This part will still be very hot. The place where the meat was has cooled down and you want all the heat you can get. The time that the meat is on the second side should take the same time as the first side so hopefully you're watching the clock. Now you are done searing and it's time to finish off whatever you were cooking.
If you have been searing a roast you need to get that roast out of the intense heat and into indirect grilling. If you've been searing chops or steaks then you want to get the heat down and finish off. So, with the lid up, turn down the heat and flip the meat over, rotating 90 degrees so you get a nice crisscross pattern. When you flip leave the meat in the same place on the grill unless you're having flare-ups. If you leave the lid up for about a minute your grill will cool down to a temperature that will allow you to get the meat cooked through without charring the surface. If the cut is thin you might be nearly done. Otherwise grill until the meat reaches desired doneness and remember to let the meat rest for at least 5 minutes before you cut into it or serve it. This allows the juices to flow back, out into the surface.
As with any method of grilling, practice is the key. If you get an over cooked chop or an overly charred steak think about what happened and adjust accordingly. No two grills are created equally and things like wind, outside air temperature, cut and thickness of the meat will change the circumstances of your sear, but with patience and practice you'll be getting that perfect sear regardless of your grill or the weather.
As for charcoal, you will need to build a two layer fire. This means that you have a single layer of coals on one side of your grill and a two layer (or more) of coals on the other side. You will sear on the hotter of the sides and finish off on the other. Depending on your charcoal grill you may not have a lot of room to work with so you'll just have to do your best. Charcoal is perfect for searing because you can get a much more intense heat from charcoal and you can control that fire much more. The searing fire is the fire you can't hold your hand over. The finishing fire should let you hold your hand over it for the count of three or four. Make sure you have this kind of fire before you start. Because of the intense heat of charcoal and the closer proximity of that fire to the meat the lid isn't going to be as important in the searing part of this grilling method.
So whether you are using gas or charcoal give searing a try. Whether you are sealing in the juices or adding great flavor to meat isn't really important. If you don't over cook you will get a juicy and flavorful piece of meat.