concentration - red bull maybe? although its not good for your body if you take excessive amount, but maybe for that one or two occasions where you need it
memory - apparently eating nuts will help improve memory over long term
but if you seriously can't remember anything, i think you have to take the liberty of memorising stuff a lot earlier than others
say if you want to memorise a bunch of quotes/equations, write them on a piece of paper in bold big letters and stick them around the house (ie in front of your bed where you can see it everytime go near it; put it behind the toilet door, so everytime you take a piss/shit you will be reading it as well)
for concepts and ideas, instead of memorising it word for word, try to understand the jizz of it, so instead of "recalling" it, you are explaining it... theres a huge difference there