First, whether you are working with pastel, charcoal or graphite, apply strokes lightly so as not to score the paper or embed the medium too heavily into the surface. This will help you when adding subsequent layers, and makes it easier to use an eraser to lighten or remove tones.
When blending, do not blend in the same direction as the stroke of the medium, because this embeds the medium in that direction and causes streaks, which may be difficult if not impossible to erase. You don’t want to create streaks that may show up under more layers of the medium.
Instead, blend in the opposite direction (across or diagonally) of the stroke. This breaks up the medium and spreads it across the surface, which makes it easier to use an eraser if needed.
Bonus tip: When trying to lighten or remove a tone, use a kneaded eraser by lightly dabbing at the area. The kneaded eraser will pick up the medium (yes, even pastel) without damaging the surface. Also a kneaded eraser can be shaped into a sharp point or any size needed for the area you are working on.