Now that the weather is nicer, it's easier to use the sled. At my gym we use it year round but inside gets boring.
We vary the pulls between heavy weight and shorter pulls (100yds or less) designed for strength and speed increase, to lighter weight and longer pulls (300-400yds) more geared towards GPP and light rehab.
Lower body it's done twice a week right now (track season). The key though is how you walk with it. When going forwards, it's really important to bring your knees up and stride out, almost pulling each step with your heel rather than pushing off the balls of your feet. It's also important to stay upright
We'll also pull the sled backwards (quads) sideways (hips) and bent over with the strap between your legs (glutes) as well as mixing other stuff in there.
For upper body it's done once a week as a rehab exercise. Walking forwards doing tricep extensions, or pec flyes, overhead press or whatever is extremely helpful in recovery. These are always done lightweight and for longer distances.
When going backwards we do rows (high and low) face pulls or curls. These are done with more weight.
The increased GPP as well as the increased recovery play a huge role in being able to train hard each week.