The best advice I was ever given was to not choose your bridal party until 6 months out. Unfortunately, I heard this advice after I'd already chosen
So I was left with a fairly last minute "firing" of my MOH and a complete reshuffle of the entire bridal party.
If you've got a MOH you're happy with, that's the biggest battle I think. The BMs don't really have much of a job, other then sharing a bit of excitement and maybe helping to plan the hen's night (which your MOH will probably do anyway). My BMs were great on the day (helping me with toilet trips, redoing my lipstick, getting people on the dance floor, etc) but they didn't really have any 'jobs' to do in the lead up.