Common strategy: Take meds temporarily, to allow yourself to concentrate for now. Stimulants aren't the type of thing you need to take constantly anyway; and who would? The side effects are so annoying... Meds can help you learn study strategies, which of course persist once you stop taking them.

Still dead-set against 'em? Okay. Are you an auditory learner, by any chance? Is there an online version of your textbook available--maybe in PDF or text format? (You may need to buy it. Yeah, I know.... ouch on the wallet.) If so, you can get a program that reads the text aloud to you. If you're very lucky, your textbook may even be available in an audio format. (Check with your school's special-needs department, whatever they call it. Maybe there was a blind student who took the class before you, and they had somebody record the text on audiotape. Doesn't hurt to ask.) Another benefit of audiotapes is that you can do a lot of things besides listen to them--keep your hands busy, doodle, whatever... I knit, for example. I've gone so far as to scan the text into the computer, use an OCR program to convert it to text, and use a text-to-speech program to read it out. The voice takes a little getting used to, though; and obscure terms might be mispronounced... it's worth a try; it helped me.