you can drill some holes in the pots towards the bottom, but depending on how slow your drainage is, you might want to drill some more higher up as well, dont go to deep to hit the roots, you just want to drill holes in the plastic. Then go get some chinese take-out, grab some chop sticks. use one chop stick to poke some holes into your medium, being careful not to kill your roots, this will help get some air into the root system and also help with the drainage. You dont want your medium to hard like the ground around your house, when you do the flush, seeing some air bubbles coming up is a good sign.....

Hope that helps