I do something similar as what Daisy recommended with my Opuntias when springtime hits. I only water my Opuntias if absolutely necessary during the wintertime, but otherwise generally avoid it. Then I give them a good soaking at the beginning of spring. I have one in particular that gets a bit floppy around this time.
Given they won't have been watered for a while, I have the same issue with getting the soil to actually take the water. I keep mine in a sink and water them slowly until the water runs from the pot (I try to trickle the water slowly so it starts to sink into the soil rather than running right down the sides of the pot). I'll repeat this a few times every 10ish minutes until the pot feels heavier, I can feel the water further down is damp, and any loose water has drained.
This works well for me, and I find the Opuntias perk back up within the next two days after the watering.
