There are some general concepts that make frequent appearances, it's worth looking out for them because they can help form connections between different areas. Some examples: 1. the harmonic oscillator and associated quadratic potential. 2. Wave-like phenomena and the wave equation. This comes up in all kinds of mechanical and em systems, plus the schroedinger equation itself. 3. Decomposition of functions into orthogonal sets of other functions, its not just a mathematical trick, but a powerful way of reconceptualizing things. 4. Approximations and expansions are everywhere. Always keep in mind what it is youre solving for and look at its sensitivity to other properties of the system.