I know this discussion has come up many times before, but I've never had to deal with it so I didn't really pay too much attention.

I usually run classic D&D (Mentzer or Moldvay) which doesn't rely too heavily on charisma checks except for a few specific circumstances.  I don't usually run games with major personality mechanics, and the few times I have, no one really made much use of them.

Now, however, a couple of my players have characters based partly or mostly on being diplomatic and persuasive in a game that supports that style of play (the new Mini Six game, a version of the D6 system), and I'm having a little difficulty deciding how to proceed with it.  

On the one hand, I decide ahead of time what NPCs' personalities are like and try to base their reactions to the PCs on that and on in-setting logic.

On the other hand, I don't want to neuter someone's character concept or make them feel like they wasted points at character creation by choosing to invest in social skills.

I don't want a good roll by a player with high social skill to give them too much control over NPCs, but I don't want to just blow off the character's high skill either.

I know others have had to deal with this before.  What's the happy medium?  How much influence should a PC have over NPCs?  Any advice?

Try to figure out a "default" disposition each NPC will have toward your character. The default would be if you are strangers during normal times. Then adjust the default based on the situation around when you meet the character. Is something going on that has them suspicious of "outsiders?" Lower disposition. Were you introduced to them by one of their friends? Raise disposition. Did you save their life? Raise disposition. Did you endanger them? Lower disposition. Etc. etc. etc.

Once you have your starting point, allow social interaction rolls to change their disposition(and what they're willing to share with you/do for you/give you/etc.) a bit. But depending on which social skill you use, there could be complications later. Diplomacy is the safe bet, so long as you keep up your end of any bargain struck. Bluff is less so; if they discover they were "had," their disposition will lower and they may even seek revenge.

I'd also suggest a soft maximum that you can raise disposition by in a single encounter. And then a hard cap one level beyond that which will also be considered "strained," as in it is very easy to lose that particular level until/unless you do something to reinforce it. Even minor slights and breaches of etiquette might be enough depending on what that person finds important.

The more negative social skills would work similarly, in that they'd increase what people are willing to do for you. I'm thinking they should have no hard limit because of their nature(unless something about the influenced NPC would make such a limit make sense). All improved dispositions from negative social skills will be temporary, requiring regular reinforcement, and "strained." And you want to be careful who you use this kind of influence on(if you let them live, of course) because when the influence wears off you could find it coming back to haunt you pretty bad. Very effective, but very double-edged sword if used on anyone who has the means to get back at you.

But yeah -- don't let PCs totally dominate NPCs with their social skills. Unless that's in-genre for the game(Exalted and epic level D&D, for example). That just makes social interactions in-game cheap.

Yes, you are absoultely right, it is tricky. One doesn't want to discourage roleplaying but at the same time one dosen't want to penalise players who spent points in social skills or limit players who aren't natually charismatic. After all isn't the whole point of rolepalying to step into the shoes of play a different person?

I go for a mix. Ultimately I let the skill and dice rolls resolve the situation, however I encourage the players to set the scene by roleplaying which can result in a small bonus (or penalty).

Note the bonus is not for the quality of the roleplaying - I don't see that as my place to judge acting  - but it's a reflection of the specific approach the player taken. It's one thing to intimidate somebody but the results may vary depending as to whether you threaten a specific NPC personally with violence or  threaten their families instead.

The way I'd prefer to do it these days is to use social skills and charisma as a general modifier.  We don't "do the voices" a lot, so if a player has his character deliver a very good rationale or convincing argument, it's likely to work no matter the PC's social skills.  If the PC is an ace diplomat, charmer or fast-talker, they'll get away with a much flakier explanation or premise.

We haven't used it much in play, because we're weaning off away from countie gaming, but it's an idea.