I would like to contribute to this thread as I have been prescribed ADD medications for the last 1.5-2 years. The ADD medication that best works for me is Dexedrine. I don't need to get into specifics as most of the posters on this thread know the advantages of pure D-Amphetamine as opposed to the mixed formula of Adderall.
For those that suffer from the dreaded "crash", I would recommend using Piracetam. I know many people balk at its claims of being able to provide clarity and cognitive enhancement, and I agree fully that the jury is still out. But, I will say this - by pure chance, I stumbled upon its effects to completely dull the crash of Dexedrine. I am pretty sensitive when it comes to Amphetamine and would suffer severe depression on doses as small as 15-20 mg's. Now, when I take my Dexedrine, I take it in conjunction with Piracetam, and I haven't had a crash since. Something to think about. YMMV of course.
Exercise, eating habits, and sleep are all key components to keeping your body in top shape on a daily regimen of amphetamines. DXM and Memantine can be used, as well, as others have mentioned. I am somewhat weary of them, though, as there really is next to nil on clinical studies of them on humans. I know they have been performed on rats and there are loads of anecdotal stories of success, but I would still be weary of long term ramifications. I will say, from personal experience, DXM does truly work at lowering tolerance for those looking for a cheaper/more convenient option other than Ketamine or Memantine. Hope this little summary helped.
Cheers