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I had an idea: use different scents for studying different subjects. That will keep everything associated better.
That's more difficult than it sounds.
"Okay, what scent do you associate most strongly with Bio 227?"
"Hm... coffee. It's always first thing in the morning, so I always have a cup on my desk."
"Okay, cool. How about Bio 210?"
"Well, I generally have a coffee and a cigarette just before that class. All through the class I can still taste the coffee, so..."
"Okay, okay, how about Chem 210?"
"Oh, my late class. Yeah, by that time I'm exhausted. If I didn't bring a large coffee with me I'd fall asleep. So, coffee."