Knots.

It's one of the first things we learn about when starting climbing. I'm not talking about the feeling in your stomach on your first exposed crux... I'm talking about the basic knowledge of how to tie in to your harness.

Lets face it, tying a knot comes naturally to some of us, while to the rest, it's just mind-boggling and can take a while to master.

Here's a site with an animated guide to tying those knots, including fundamentally imporant ones like the figure of eight, plus others thay may come in useful some day! The italian hitch did for me when I dropped my belay device on a multi-pitch in Omis, Croatia... oops!

Remember to practice, especially if you're a beginner at this, and always have a more experienced climber friend check it for you until you're confident. Confidence in your knot-tying gives you as a climber or belayer one less thing to worry about, and more ammo to send that route strongly.