When working in Premiere Pro or just about any other Adobe application, it's very clear that being able to move nimbly between panels is a necessity. Although using the mouse does work, it can be pretty cumbersome. Here are the default Adobe keyboard shortcuts for the application panels:
Shift + 1 Project Panel
Shift + 2 Source Panel
Shift + 3 Timeline Panel
Shift + 4 Program Monitor Panel
Here's a link for all of them.
However I jump to and from these panels frequently so I substituted single-stroke keyboard shortcuts to make things even simpler.
Here's the link to Adobe Knowledge Base article on customizing keyboard shortcuts. Note that if you attempt to assign a key that already in use for another shortcut, the program will alert you to the conflict. From there, you may choose whether or not to reassign that key.
Here are the substitutes that I use:
F8 - Effects Panel
F9 - Source Monitor Panel
F10 - Program Monitor Panel
F11 - Project Panel
F12 - Timeline Panel