I'm by no means a pure functional programmer, but I've found some of the mantra from that paradigm helps when building the early blocks of a program as it allows you to break down your application into core problems which can be tackled individually. Plus it allows your application to scale easier and makes it easier to rip out and replace code as you start to add complexity.

But of course, before starting any major project, the first thing any developer should do is map out at least a basic mental design of the program - even if that map isn't written in pseudo-code or in a form of a flow chart.