The most important thing I had to learn when I started on hardcore ANSI C projects (large embedded business critical application) was to really learn to build a

The most important thing I had to learn when I started on hardcore ANSI C projects (large embedded business critical application) was to really learn to build abstractions. C has few tools to help you with this and so it is super important to get the most of what is available. Another lesson is that idioms are very useful in C and cut a ton of time. For most or all repeatable tasks there should be conventions for how to implement it and how to use it. These are useful in any programming language and environment but I think are especially useful in C where you are on your own when structuring your application.