Study of the concept of application for the microcontroller

With the increasing complexity of our projects (in General, an embedded system), growing complexity of software for microcontrollers. It is difficult to schedule, difficult to predict the complexity, the harder it is to divide the task into subtasks. If earlier the program was written in a day or two, and I kept in mind, that now the volume of work requires weeks and months. For example, the last project I was involved in a full GUI on TFT-touch screen, a small web server, telnet console, CANopen stack, the file system on the external USB flash drive etc. this was All written in normal si cortex-m3 microcontroller. In the concept phase, I used anything from the usual algorithms to different kinds of diagrams. All of this helped a little. I would be grateful if tell me how you can apply proven methods of concept development software. I tried to use UML, but it fell into perfect confusion. Class diagrams? I have a regular si, it's not even C++. Charts precedents? Perhaps, Yes, but then what? How and in what order to apply these digrame how to plan the development process? And finally, what programs to use?