I have always maintained that I would never charge membership to our site.
If I was to go back on that now, I would get quoted and fried alive!
What I could do is offer some sort of fellowship option where you can opt to pay 2 pound a month or something?
I'm a member of various forums across the internet, a few of which offer a 'supporters' package, which is a small amount, say £10-15 a year and has some useful options, but doesn't restrict any access for the free accounts.
Things like a bigger inbox for messages, or more storage for photos - so things that make the forum easier to use - rather than linking from flickr, or having to delete old photos to make space with the limited account - as I say, the aim is to provide an easier version of the free experience, but not by preventing free members from doing anything.
Some places tag the users as supporters too, on the left below the username, instead of Senior Member / Forum Owner / Moderator etc.
So, those who visit and use regularly would benefit the most from the 'premium' membership, but the experience and knowledge can all still be fully accessed everyone.
I've seen this expanded to include club discounts / access to supporters only area / access to classifieds
BUT in my experience, only the former really works.
Classifieds just die away due to limiting those who can access them, and supporters only area just does what you are trying to avoid, and starts to move towards this becoming a subscription service. Club discounts can pay for themselves, but potentially involve you having to chase down retailers to get the discounts in the first place, which is another PITA, and perhaps not quite in keeping with the spirit of this place.
Finally, the option to remove ads, but then you don't want intrusive ads in the first place (I presume SM has ads? I use a blocker...)