I've come up with three possible stipulations for being able to claim territory:
1) you must have one of your units in the territory, at least, preferrably
2) you must claim a territory with a special type of unit such as a scout, explorer (then perhaps they might be worth producing), great leader, worker, and/or settlers - I'm not sure which would be best to use for this, but it would have to be limited.
3) the claimed territory must be within a certain distance from one of your cities, colonies, or a garrisoned fort/barricade. This would make it a bit more more difficult to claim territory. Perhaps have a ten/five/three space distance from cities/colonies/garrisoned fort respectively.
Then once you hold this territory it remains yours unless you:
1) sell/give it to a rival nation,
2) another nations culture supercedes your claim,
2) it is militarily seized by a rival nation - causing war, or
3) it is abandoned to a rival nation - avoiding war.
This does make it fairly easy to capture and maintain enemy territory, but thats just another factor I think would limit liberal land claims. I think it would be interesting to allow culturally held to be militarily captured without taking the rival city. Perhaps cultural territory could be captured if an enemy nation leaves a garrison on the territory for 3-5? turns, but if all military units leave, the area immediately reverts back to the culturally dominant nation. That was a it would be possible to resources near your border without having to actually capture an enemy city - of course you'd have to declare war to do this, but the effect could last after a peace had been negotiated as long as you retain your military presence. Maybe not the greatest idea, just trying to throw ideas around.