To calculate input resistance, it's essential to avoid guessing formulas and instead apply reasoning based on the circuit's characteristics. The input impedance can vary depending on the frequency, so it's crucial to clarify whether a specific frequency or a general range is needed. Simplifying the circuit by ignoring negligible components can help in calculations, such as treating 10KΩ as dominant over 1mΩ. When dealing with transistors, using a simple AC small signal model can clarify the analysis by identifying components that do not influence input current. This method allows for the creation of a streamlined schematic focusing on the relevant elements for accurate input resistance calculation.