Know the product!
The principles of mechanicing are all the same. Knowing the finer details comes from experience on a specific product. You need to learn the difference between the different models of vehicle you are working on. Once you know the different makes and models you can then drill down into it.
What are the different engine oil capacities? if you know this it will save you time looking it up in the workshop manual or fucking around adding oil Litre by Litre, checking the dipstick trying to get it right.
What are the different tools you need to service different models? What different tools do you need to do this task between models. I always think to myself. If I had been working here for 20 years what would I know. Then I work hard to close that gap..
Create a list of Makes and Models that you work on. Then start drilling down into each different model. Write the different engines, g-boxes, oil capacities, sump plug sizes, different identifying factors, etc. etc. you get the idea. Know your product inside out