If you're serious about getting everyone signed up early, maybe try offering something to the teams that get signed up/paid early, because it seems that threatening the season/not extending hasn't worked well in the past.
Say every roster with 6 paid a week early could get some 1 week premium codes or let them get a backup "paid" for free, or just a lower rate - that might a bunch of folks to actually sign up before the last minute.
As it is, there's no incentive to not wait for the last minute.