I think part of the problem is that certain terrain improvements (like colonies, outposts and forts) should actually impose a cultural border if they are protected by your military. This way, the only way for a neighbour to destroy your claim to that piece of land would be to conquer it-or purchase it from you. As an added effect, perhaps these improvements should have a max. no. of troops that you can station there. So a colony might have 2, an outpost 3 and a fort 4. For each unit you station, you expand the border out by 1 hex, in each direction (note HEX, not square
!). This way, you have a temporary way of holding on to territory near an enemy border, which doesn't cost you 2 population points! Of course, you would still be better off eventually building a town there, 'cause your borders won't grow any greater than 4!
This brings me to another point. Though I like the connection between culture and borders, this should NOT be the whole story. THe system I mentioned above opens the possibility of 'Annexation', but it should also be possible to sell, trade or demand land within diplomacy. A simple click and point system, on a minimap of your controlled territories, would allow you to highlight the areas you want to claim and, once you've done that, you can go to the table and tell them WHAT you want for it!
Another way to claim land, that I have considered, is explorers. Perhaps an explorer can actually have a 'Claim Land' or 'Plant Flag' mission. This will create a 2x2 (or 3x3) culture border centered on the explorer. As long as the explorer, or a military unit, remains within that border, then it cannot be subsumed by another party (of course, killing the explorer or a defending unit will make it revert to neutral). Such claims will be recognised by allies, however, and you would be able to sell it as THOUGH it was yours (though other civs might dispute your right to do so
)!
EDIT: Another, more simple possibility, is to give the explorer the Build 'Colony', 'Outpost' and 'Fortress' terraforming missions.
Aussie_Lurker.