Direct debits. Marvellous things. ORG (openrightsgroup.org) started with getting pledges of £5/month to get it started. They have, like, money and stuff now. Not buckets, but certainly enough to work with.
Payroll giving would be even better than direct debits in many cases. We should certainly set up direct debits if we can. From the little bit of research into bank accounts I've done so far, it looks like we may have to pay fees for direct debits even if we don't have to pay them normally - I have no idea what those fees would be, but it sets a minimum amount it's worth having people pay - £5/month should be fine, though. Membership fees would be great if done through direct debit - stops people lapsing through inaction.