There are the obvious Skill Focus (Bluff) (+3 to Bluff), and Deceitful (+2 to Bluff). The ultimate intrigue also brings Blustering Bluff, which allows you to basically re-try to tell a lie telling another lie and decreasing the DC of the check.
There is a fair amount of traits that grant +1 to bluff, you can check those here (just filter by "Bluff").
There are two cheap and useful items to obtain a quick bonus on Bluff checks:
Blue Book: By consulting the book for 1 hour, for the next 24 hours you gain a +2 circumstance bonus on Knowledge (local), Bluff, and Diplomacy checks to gather or use information in that city.
Perfume Kit: Applying scents from a perfume kit takes 1 minute, and grants you a +1 circumstance bonus on Bluff, Diplomacy, Disguise, Handle Animal, Intimidate, or Perform checks, subject to GM discretion.
The Mask of Stony Demeanor may be too costly at your level (8,000 gp), there are a few others that could be helpful: Ring of Eloquence, Veil of Veils, Cracked Mulberry Pentacle Ioun Stone, and the Honeytongue Elixir.
Honestly, there are better classes to pick when the point is to make a good bluffer, such as bard, investigator, or even a Spy Rogue. But since your choice is already set, there isn't much we can do here. Except for Rogue Talents.
Rogues have quite a few abilities to improve their Bluff skill usage.
- False Friend gives +4 to Bluff checks against those you never met before.
- Honeyed Words allows you to re-roll a Bluff check once per day.
- Coax Information allows you to use Bluff to intimidate someone to act friendly towards you.
- Convincing Lie is a tricky one. Basically it makes people use your own bluff modifier when passing along your lies, basically spreading them like wildfire. So, if you are a truly exceptional liar, no one will ever question it.
- False Attacker allows you to trick someone who you just backstabbed from the shadows into believing that someone else did it (no, seriously, take a look at it).
- Innocent Facade allows you to use the Innocense spell once per day, which grants a +10 on Bluff checks to convince others that you are innocent.