One suggestion: allow members to sign up for multiple years in advance. I'm perfectly happy to pay for two or three years of membership in one go.
There may be some practical issues, such as whether or not if one were to, say, sign up for a three year membership but resign after a year, the remainder would be refunded as unused.
If it were offered, I'd certainly be happy to pay for a few years in advance so that I don't have the mildly irritating situation of wanting to run some kind of outreach event and not being able to apply for a microgrant because my membership has lapsed.