Basically, put yourselves about a bit....
First off, do you have a notice board on or near the gate(s) with contact details? If not, that's number one priority.
We affiliated to the Borough's Allotment Federation, so our details - and again, my contact details - are available on their website. (You could, of course, get your own website, but that's not for starters.)
We invited in the local primary schools (and their mums); put up an exhibition, with flyers, in the library; made friends with the Residents' Association - I write an occasional article for their magazine - and the Chamber of Commerce; we have a plant sale - again with flyers to the fore - at their annual Fun Day. And we're very much to the fore in community matters and meetings, so that people all know we actually exist.
All of this takes a lot of time and effort, of course. But a notice-board on the gate is easy.
