It strikes me that are a lot of very knowledgeable people here and sharing some of that knowledge when it comes to preventative maintenance may not be a bad idea.
So post up your maintenance tips and checks that you do to both, spot problems before they happen, and also help prevent them happening.
Here's some of the stuff I do:
1/ Rotating Wheels - Brake pads & ball joints
When rotating the wheels, I look at the brake pads and check for wear. I also grab the front tires when they are off the ground at the top and bottom and give them shake, checking for free play in the ball joints. When it's back on all fours, grab the drag link and tie rod and give them a good shake and see if there is any free play.
2/ Oil Change - Lube
When doing an oil change I also lube all the grease points. I use a needle adapter for the ball joints and rod ends, etc, that don't have a grease fitting.
3/ Oil Change - Fluid levels
It takes just a minute or so to check the PS and brake fluids, and the transmission fluid while you have the hood open to add new new oil.
4/ T-Case & Diffs
Every 10k miles or so I check the t-case and diff levels and fluid condition. Check the diff fluids after anytime you've been in deep water.
5/ Torque Settings
Every so often and after every wheeling trip I check the torque settings on the control arms, track bars, etc. Check for signs of bolts moving (worn surfaces, etc) as you may have elongated holes/mounts, especially if the bolts were torqued correctly.
None of the above takes long, and is well worth doing. You can spot a lot of potential problems early if you take a few minutes every so often to check things out.
If you rely on others to do you oil changes etc, make sure they actually do lube the points they are supposed to, as my general experience is that they don't do it, even when they've said they did.