I've been doing math for a long, long, long time. But I still make "stupid mistakes" such as dropping a minus sign, or forgetting to follow through with the chain rule in differentiation. I'm not even very good at basic arithmetic.
I guess part of the problem is that I never had to work hard at getting the right answer in school, so I never practiced a lot. I've also been diagnosed with ADHD, dysgraphia and other learning-related disabilities.
In one of his lectures Dr. Leonardo(sic) Susskind stated that he was copying his results from his notes because if he tried to do the work on the fly, he would just make a bunch of algebra mistakes. I say this to point out that even people of august standing in mathematical fields have problems executing basic manipulations flawlessly.
One approach to improving my skills might be to start with a first grade math book and work through all the problems assigned to students from K to 12. It's something on my bucket list, but I haven't gotten around to it.
In lieu of repeating 12 years of pre-college math, are there any suggestions for improving symbol manipulation skills? I know, and believe the old adage practice makes perfect. I'm wondering if there are any kinds of focused exercises which efficiently address mechanical math skills.