The best time to get a squirrel sitting still is when they are occupied with eating. So long as you don't approach them to suddenly they will basically ignore you while they munch away and its neat to get them standing upright and so very focused on their feast that nothing else matters.
Nikon D750 1/160 sec. f/5.6 ISO-500 at 500mm
Nikon D750 1/160 sec. f/5.6 ISO-500 at 500mm