Traits are your best bet. The feat "Additional Traits" can be used if you're already past character creation, provided you still have the correct trait slot available.
There are items that give you a bonus. Is there any reason it has to be made into a Class skill? Wouldn't any +3 be just as good?
Well, making it a Class Skill gives the +3 bonus, and then any additional items will give additional bonus. I'm not looking for the ultimate Diplomat but any reasonable way to increase the skill will be considered.
There are a couple of spells that let you treat yourself as trained in a skill and a number of buffs that give Insight bonuses to all kinds of skills. Bestow Insight, for instance. As a bonus, it lets you roll twice and take the better result if you need to pass a Diplomacy roll. On the other hand, with a sufficiently high CL, True Skill becomes the superior choice for an Insight bonus.
Similarly, Embrace Destiny, if you have a bloated CL or are able to anticipate when you'll need to make the check and only need a brief window of time but can choose your window of time in which to make the check, can let you burn spell slots in order to guarantee a good die roll.
Visualization of the Mind allows you to either A. give yourself an untyped +5 to Charisma checks and Charisma-skills or B. give yourself full ranks in Diplomacy but only for one minute.
Bit of Luck can be used to give yourself a potentially significant Luck bonus on a few specific skill checks.