Rehearsal. Now, let me see... Trying to get all band members to attend regularly on the agreed schedule.
Once other band members have to'look after the kid(s) this week', go on holiday ,
or are very sick the momentum collapses. (only my experiences tho')
I think regular rehearsals are needed,so that all band members can see how far they've progressed with the songs (or not).
If you're of a higher standard ,then regular practice is not needed as much imho