(Player's Handbook v.3.5 variant, p. 71)
Use this skill to persuade the chamberlain to let you see the king, to negotiate peace between feuding barbarian tribes, or to convince the ogre mages that have captured you that they should ransom you back to your friends instead of twisting your limbs off one by one. Diplomacy includes etiquette, social grace, tact, subtlety, and a way with words. A skilled character knows the formal and informal rules of conduct, social expectations, proper forms of address, and so on. This skill represents the ability to give others the right impression of oneself, to negotiate effectively, and to influence others.
Optional, but not recommended because retries usually do not work. Even if the initial Diplomacy check succeeds, the other character can be persuaded only so far, and a retry may do more harm than good. If the initial check fails, the other character has probably become more firmly committed to his position, and a retry is futile.
A half-elf has a +2 racial bonus on Diplomacy checks, thanks to her ability to relate well to others.
If you have the Negotiator feat, you get a +2 bonus on Diplomacy checks.
If you have 5 or more ranks in Bluff, Knowledge (nobility and royalty), or Sense Motive, you get a +2 bonus on Diplomacy checks.
Required for feats