Its a lot easier to see patterns in monitoring data after an eruption has occurred. But great progress has been made because of the lessons learned over many years at Kilauea volcano in Hawaii, and applied and modified at Mt. St. Helens before and during in eruption sequence in the 1980s. VolcanoWorld is constructing a series of pages concerning volcano monitoring and prediction, but since they will not be ready for some months let me give a few opinions.
Meaningful prediction requires careful monitoring of a volcano's vital signs.
Seismometers can be used to pinpoint earthquakes which track the rise of magma and its movement along fissures. Instruments that sniff SO2, CO2 and other gases also can signal changes in the volcano. At some volcanoes the seismic information seems most reliable, at others the tilt tells the story. But the best predictions come from the combination of all of these methods into a volcano monitoring and prediction system. And you must remember that each volcano is unique.