For steeper slopes, you may not be able to keep your center of gravity ahead of the rear tire while still sitting on the saddle. Move your body forward so you're positioned straight over the crank. If you go too far forward, the rear tire may slip. Too far back, and your front tire pops up.
You need to keep some weight on the handlebars (the more, the better). Feel yourself "pushing up" the slope with your hands, not "pulling up." In other words, if you're pulling back on the handlebars as you climb, you need to get forward!
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