Antiperspirants work by producing a salt compound that physically blocks pores, and this chemical process works better when skin is completely dry, says Pariser (moisture causes the chemical process to take place on the surface of the skin, not underneath it). We tend to sweat less before bed, which is why all antiperspirants are more effective when applied at night. You don't even need to shower first, says Pariser; but if your armpits are damp, you can use a hairdryer to make them completely moisture-free. Pariser says the plugs will stay put for around 24 hours, even during a workout and a rinse.
One more thing: If you need extra reassurance before going to work, swipe on some deodorant. It won't help fight wetness, but it will mask odors.