We generally recommend using a few drops of dish soap to increase the wettability of seed-starting mix (which is often dry and hydrophobic right out of the bag). In that case you can tip the potting soil into a bucket and let it soak overnight to take up water to saturation - and that might also be your solution here. 

Before you re-use the potting soil, consider its history in addition to its moisture status. Commercial potting soil is generally a mix of peat, vermiculite and composted matter with slow-release nutrients added during the manufacturing process. Soil in containers is often depleted after a growing season because plants use up some nutrients and others are leached out by irrigation. Container plants can’t send roots deep below the surface to source minerals and they are lacking the natural soil ecosystem that would break down organic matter and release more nutrients. Viruses, fungi, and bacteria also live in the soil long after the demise of a plant, so if you had disease problems in the containers they are still around.

If the previous occupants of the containers were disease-free, you can add slow-release fertilizer and some fresh potting soil to the surface to plant into. If you’re investing in a whole new plant project this might be a time to add the used potting soil to your compost pile and start over. Finished compost can be used on garden beds or for topping up containers later in the season - it doesn't go to waste.