I'm glad you asked that, because we added a really cool feature to BaseCamp that will automatically filter a track to a specified number of points during the transfer. Go to Preferences->Transfer and check the "Simplify tracks by removing intermediate points" check box. Change the number of points to the max number for your GPS; the default is 500. Now when you transfer the track, it will have the right number of points for your GPS.
Note that the filtering is non-destructive to the original track. It only filters the track as it is being transferred.
There isn't a way to filter the track in the application directly. An indirect way is to transfer the track to your unit with the filter option enabled and then read the data back.
