All these quotes are a bit pedestrian to attract fresh blood. What you need are giveaways, freebies... the lure of a prize.
Needs to be audience specific, so got to be something that would only appeal to cavers. How about a free bucket of mud for every 1,000th post on the forums? *
* Over 18s only, not exchangeable for cash, mud to be claim in-person only, offer not open to residents of the Isle of Man, no liability in the event of parasitic disease or injury. Bucket remains sole property of UK caving.