I'd say that I'm in the minority when it comes to pulling trailers. I pull the same trailer all the time and rarely unhook. Keeping up with the maintenance on it is a cinch as I know everything that's happened to it and have personally been involved with all maintenance. However, most drivers are involved in a drop and hook operation and with each pre trip, they wonder what kind of surprises they're going to find. Tires and lights are the first things most drivers look at and rightfully so. But don't overlook the brakes, especially if you travel the mountains!
Most newer tractors have self-adjusting brakes so there's nothing to do there but maintain proper lubrication for the system to work. A proper lubrication schedule is sometimes overlooked, but extremely important as it will prevent most systems from early failure. Bendix, one of the largest manufacturers of brake systems recommends that you check adjustment on manual slack adjusters every month, 8000 miles or 300 operating hours. You can check out their service bulletin here. If you're pulling different trailers from day to day, you don't know when the last time the brakes were inspected so it's up to you to do it. 
Firstly, make sure your trailer brakes are released (keep tractor brakes engaged) and wheels chocked. Then climb under trailer and position yourself under each brake chamber. If you have a car hauler, lowboy or RGN (removable goose neck) the adjustments are usually made from the top of the trailer so it's something you do while laying on your stomach looking down. Locate the adjusting mechanism on the slack adjuster. It usually takes a 9/16 wrench to turn it. Tighten it all the way; you should see the S-cams move and the brake shoes tighten against the drum. Then, loosen it 1/2 turn and you should be good. Some mechanics loosen it 1/4 turn, but I don't want to take a chance of the brakes being too tight and dragging. The DOT inspector will measure the push road coming out of the brake chamber and if it's more than 2", you'll be out of service until they're correct. If you can do what I"m telling you in this blog, you can make the adjustment and be on your way. Turning back a 1/2 turn usually puts the push rod at 1"of travel which is the optimal adjustment and well within the tolerance of 2". If you have any more questions, check out this Youtube video that explains it pretty well.