Grunt UphillsGoals: (1) keep the front tire on the slope, (2) keep traction on the back tire, (3) deliver power to the pedals
Done by: (1) body forward, (2) center of gravity over the crank, (3) push down on the handlebars
As you approach the slope, decide on your climbing gear. The best gear isn't always the same: it depends on your speed coming into the slope, and the steepness and length of the hill. On some slopes, you'll be able to hit the hill fast, in a high gear, then downshift as you lose speed. On others, you need to drop directly into your climbing gear before you hit the hill. With experience, you'll learn how to gear your bike for a climb just by eyeballing the slope.
When climbing a steep uphill, keep enough weight on the front tire so it stays on the surface. If the wheel pops up, you'll "wheelie out of the climb" or tip to the side. The goal is to keep your center of gravity forward: ideally, directly over the crank, but certainly ahead of the rear tire. For a mild slope, a simple forward lean may be all you need. Keeping your butt on the front tip of the saddle, move your chest as close to the handlebars as you can. If you can stay seated, you can keep power to both pedals through the entire stroke -- assuming you're clipped in.