Lord Vangarel, the biggest suggestion is to ensure that silver-tongued players do not overcompensate for uncharismatic characters. Too many DM's put more weight on what a player says during roleplaying an encounter than on how the character would have actually done it. It doesn't matter if Bob the Player can sell refrigerators to Pit Fiends, if the 5 charisma, 0 diplomacy rank fighter he plays tries to convince someone of his point of view, it's going to be a difficult sell. In my opinion, if a player playing such a socially-inept character is trying to overcome this, THE PLAYER IS ROLE-PLAYING POORLY.
I highly recommend renting the Martin Lawrence movie
Black Knight. In addition to being hillarious and a fun time, it has one scene in it of a TERRIBLE diplomacy check, followed by a failed diplomacy check from another person, then a SPECTACULAR diplomacy check as a re-try. You will know it when you see it.
The main point is that if you make it necessary, then your players will see it as necessary.
here is a suggestion: A local noble has a mine that has been infested with monsters. He needs a party to clean it, but NOT JUST ANY party will do. Who knows what shady hooligans are going to tear up his equipment and steal him blind worse than the monsters??? The place is a goldmine of wealth in monster parts and treasure, but unless they can convince the noble to give them the job, they aren't legally coming within 1 mile of the place.

The party may want to hire an NPC bard just to convince the noble!!