I usually:
But how I can jump back to NERDTree to open one more file in a tab?
Temporary solution I use now in my .vimrc file:
map <F10> :NERDTree /path/to/root/of/my/project
But it's not very useful to start navigation again and again from the root directory.
Ctrl-ww
This will move between open windows (so you could hop between the NERDTree window, the file you are editing and the help window, for example... just hold down Ctrl and press w twice).