I discussed professional geologist (PG) license application issues a while back, but I never got into detail regarding the piece that I think intimidates applicants the most: the ASBOG test that is used in most US states.
I found a long exchange here regarding exam preparation, but here's my take, as someone who passed both parts on the first try (yay!):
You can often find a local workshop/weekend course that will give a good overview of the exam contents and testing strategies. In my case, my schedule didn't match with the review course offered locally and I just went with a study manual and practice test book.
I used the study manual more as a check to make sure that I knew the topics covered and to note gaps in my knowledge. For those subjects that I had missed in undergrad/grad school, I borrowed a couple of
textbooks from coworkers. However, I had zero experience, academic or
otherwise, in resource geology, and nobody else seemed to have a
textbook I could borrow. So I ended up getting a laughably old textbook
out of the library.
The practice test book only has two of each practice exam, so I used the first two as an assessment and the last two after I'd done a bunch of studying and felt like I'd filled some gaps in my knowledge. In both cases, I tried to follow exam protocol (no distractions, timed, etc) so that I had a representative experience. In my experience, the practice test questions were harder than the actual exam, so even though I narrowly missed passing one of the "post study" practice exams I passed the actual exams handily.
I am a good test taker, but not a memorizer. So I spent a lot of time going back to some basic stuff that I just don't work with on a regular basis, like the geologic time scale and rock QAP diagrams. I also thumbed through a geological dictionary to make sure my geological vocab was up to snuff, and pulled together a long list of equations (and symbol explanation list) for last-minute review. I tried to spend at least an hour every night on problem sets for the month before the exam.
For the actual tests, I went through and did all the easy questions (those with no diagrams/calculations) first. I noted the answers that didn't take any time but which I was unsure of. Then I went to the calculations I was reasonably sure of, checking to make sure that I had my units straight and that I knew what, exactly, they were looking for. Then I did the calculations I was less sure of, and then I went back to those complicated "look at this puzzle of a geologic map and tell me what happened when" questions. I finished the practice of geology exam with time to spare, but didn't have time to re-check any questionable answers for the fundamentals of geology exam.
It can be lonely preparing for the exams, especially if you're in a small organization or one in which everyone else has been grandfathered in. But it is gratifying to actually sit for the exam and meet all sorts of other geologists who are in the same boat - and remember, most people pass eventually!