• if something feels too "heavy", like it's doing xml formatting, file manips, a db insert, and making coffee, all in a single class or function

Separate out those "concerns", into their own object/interface, and pass them into the class / function at invocation (Dependency Injection)

• use "if guards" and early returns to bail from a function, instead of wrapping the func body with an if

public Value? Func(String arg) {
  if (arg.IsEmpty()) {
    return null;
  }
  
  if (this.Bar == null) {
    return null;
  }

  // ...
  return new Value();


  /// instead of

  if (!arg.IsEmpty) {
    if (this.Bar != null) {
      // ...
      return new Value();
    }
  }
return null;
}