Playing 1.8.1e but influence is useless to me. First of all, I have much, much more of it than I need. Second, all my lords give immediately their prisoners away, to get influence, which means prisons are full and leaking.
How do you get influence? In votes (see below), and I think generally you should get influence by being wise and strong leader, and having good relationships. Have high tier clan, towns that are prosperous and loyal, castles and town forts that are well manned, have vassals, be rich (relatively within the kigdom), marry high influence clan members (even if your actions would reduce your influence, these would slowly build it up again). Also aiding someone in a battle, doing quests, giving money & items.
Yeah I agree there should be only certain amount influence available. In votes, you would not actually spend influence, but influence would attract other lords to join the proposal one lord makes. If the lord is very influential, he probably gets backed up by many lords. Others would oppose the proposal. If as a king you agree with the proposal which has more support, your influence increases, and so does the influence of the lords who supported the winning proposal. If you support the less support proposal, your influence reduces. If you're a vassal, your influence grows or reduces based on if you supported the winning proposal.
How much influence should be in a kingdom? I think it should depend on how many clans (or lords) are there. Influence of a clan (or a lord) would never drop to zero nor rise sky high, but it might change let's say, between 10 to 50 points, for example.