As a rule, untuned offsets have a simple convection pattern from firebox to flue. Almost anything you can do to break up the natural pattern, will help your pit distribute heat more evenly.
Don't bother unless the heat distribution is so uneven you have to rotate your food every couple of hours or the meat itself cooks unevenly side to side. In other words, if it ain't broke, don't fix it.
The first step for any size offset regardless of size is to put a baffle in the cooking chamber over the opening to the firebox. The second step is to drop the flue so it's at least as low as cooking grate height. If you don't know what I'm talking about take a look at http://www.homebbq.com/library/SmokerModifications.pdf
All the recommendations there are gold.
In small offsets, water pans work very well. And the combination of a water and drip pan is usually enough for a very good tune. You can place the water pan near the fire box bulkhead where it'll block some of the radiant heat coming, and provide a cooler thermal mass, in what was formerly a hot spot. My pit is tuned to within 10 degrees with a water pan and a drip pan (convection confuser). 5 degrees when the loaf pan has been freshly filled. In a small offset I'd spend a lot of time fooling around with two pans before I moved on to tuning plates. In a medium offset or larger (20X40 and up), already equipped with a good drain, you're probably going to need plates.
You want a water pan with a low surface area relative to its volume because you want to humidify the air -- not saturate it with steam it can't absorb. It needs to be large enough so that it doesn't force you to open the chamber door more often than you otherwise would. I use a cheap steel loaf pan, and get three hours out of it. Spot it on the cooking grate, parallel to the bulkhead (to block radiant heat) above the baffle (block more radiant heat), close to the bulkhead (cooler thermal mass in a hot spot). Tip: Start and refill your water pan with hot water, and your pit will come to thermal equilibrium quicker.
A full size (#1) steam table (aka hotel) pan makes an excellent drip pan that will last forever if cleaned occasionally. The drip pan goes directly below the primary cooking area, on the lower grate to catch the drips. Some people find that a little water in the drip pan at the start of a cook helpful. Others find it best to work with a dry pan. Trial & Error.
A water pan is a good idea in any offset most of the time for a bunch of reasons. But at a certain size smoker, the best a water pan can do for you in terms of tuning is give it a little shove in the right direction.
"Quarry tile," aka "saltillos," aka "those kinda Mexicany-Mediterraneany red terra-cotta floor tiles" work well as tuning plates, as well as steel, They have high thermal mass, can be cut to fit easily (with a tile saw), and double as baking stones. The standard is steel plates. You want to use something fairly heavy guage. 3/16" is a pretty good stock size.
Cut four plates or tiles 2" shorter than the width of your cooker at 5", 4" 3" and 2" wide. Start by playing with the 5" and 3" plates and moving around your lower grate to see what you can do. Add more plates and rearrange as you begin to understand the flow pattern. You may need more plates. Once they've got the pit tuned, some people drill out the corners of the plates so they can be wired down to keep them from shifting.
Wind can play havoc. It's nice to be able to turn your pit so the door isn't directly into the wind. It's a good idea to get a diverter for the top of the flue, which you can rotate. There's not much to lose by bypassing a flapper because most of us keep our flues all the way open when burning charcoal or sticks for heat. You can get "T" shaped diverters at big-box hardware stores and fireplace stores. No matter what you do, big wind will draw a lot of air and detune the flow in your box. It's one of life's little pleasures. What can you do?
Hope this helps.