It's great that you want new membership! I didn't even know there were Orchid Societies in my area. I think that promotion will play a large part in your rejuvenation efforts.
Make sure you have contact information on every piece of information posted. There's a garden society here in Toms River and every time I go to the Library, I see a small sign they have there. However, there is NO contact information, and even the librarians who put up the sign don't know how to get in contact with anyone from the society. I think that's weird, as if they don't *want* new membership.
Networking is a great way to drum up new membership, but it limits the possibilities. I am approximately 20 years junior to the members of the society in my area. Had I not become interested enough to seek them out on my own, I wouldn't have networked with them as we don't have much opportunity to have common ground. I have a 2 year old, their kids are grown, and things like that.
In advertising for new members, the existing members may want to go outside their comfort zone to place flyers, and to go to age/gender/race/sexual orientation diverse areas. For example, with the society of which I speak, PTA meetings are a thing of the past for them, but might be a great place to inform potential members. Not to mention networking at Business Networking meetings, Chamber of Commerce, etc.
Community Education might be another way to reach new membership. Our library has programs all the time, on a wide variety of topics. It doesn't cost anything but time and supplies for the presenter, and people are often willing to attend free stuff, especially if it's at a time when their kids are at another program there at the library. Scouting troops have badges oriented on plants, and lots of moms/dads make arrangements to be there for the presentations if they are going to be of interest to them. Lots of other organizations bring in presenters as a treat for their members, too.
Also consider availability of meetings. I know consistency is important, but if meetings are usually a weekday evening, perhaps rotating every third meeting to a Sunday afternoon might allow folks with other activities to attend.
And I agree about not shooting down the enthusiasm of new members. Nothing is worse than wanting to get involved and DO things and being shot down or dragged down by naysayers. Help channel their enthusiasm and pair them up with dynamic members of the old guard. I know new members often want to take on huge projects, teach them to scale the project to a mangeable task, with plenty of follow up and follow through. If you don't give them *new* projects with which to get involved, channel them into existing projects, even just welcoming members at the door. It definitely helps you get involved if you feel useful.
just my