Precautionary measures that can be taken before and after a volcanic eruption

The most effective precautionary measure is to move far away from any active or dormant volcano, barring that, other precautionary measures for before a volcanic eruption is to constantly monitor the volcano for activity and to have an evacuation plan in place in case the volcano does erupt.
During an eruption it all depends on what type of eruption it is. If it is a cinder cone, just staying away is good enough. If it is a lava floe, you can just walk away, most lava moves pretty slowly, do not ever walk on the lava, it may look solid but often has molten lava running beneath it. Stay away from where lava meets water, there can be explosions as the lava cools quickly. If the Eruption is a pyroclastic flow, the only thing you can do is pray that the pyroclastic flow runs out of steam before it reaches you. Pyroclastic flows can move at several hundreds mph and at temperatures of hundreds of degrees Fareignhiet.
Also in any type of eruption stay away from any streams that flow from the volcano, you are at risk from a type of flood called a lahar. Lahars are fast flowing rapid overflowing floods filled with all sorts of debris, and if the volcano has a glacier on it, the entire glacier can melt adding to the floodwaters.

Afterwards it is best to stay away from the area affected by the volcano. Poisonous gases can rise out of the ash and any lava may not be completely solidified, you could fall through. Sometimes there are pockets of gas buried in the volcanic ash, so don't dig through it, the gas will usually work its way out, but that could take a long time.