Books are the Key
Find as many books as you can on the subject of Gunsmithing and read them cover to cover, many times. Build a good reference library on the many different types of firearms so you can identify by sight or if not from memory, a place you can look to try to identify the firearm in question. Once you know what you are working with, half the battle is won.
Never try to disassemble a firearm without looking at its exploded diagram or some reference book that has disassembly instructions. There are many guns where if a lever or other part is not in the correct position for disassembly, the gun will "lock-up" and many, many hours are required to fix the mistake. Fore knowledge can save many hours of needless labor. For a start, Gun Parts Corp. has a two volume set (under $25) which shows exploded diagrams for hundreds of different firearms.
I been gathering and collecting firearm books for almost 50 years and I think I have several hundred gun books in my library.
Being ROAD KILL on the Information Super Highway and Electrically Challenged really SUCKS!!
Every problem can be solved through the proper application of explosives, duct tape, teflon, WD-40, or any combo of the aforementioned items.