[edit] Steps

Cut off the bottoms of all your pants. When people ask you about it, claim that you snagged them on sharp pieces of coral.
Find a puffy shirt and wear it all the time...if it was originally white, it should turn a nice beige/taupe color. Add a contrasting sash. If your job forbids you to openly dress like a pirate, wear the pirate gear under your business suit (uniform, mascot outfit, etc.). When people begin to wonder about the stench, tell them it's the smell of the sea.
Integrate the following words into your lexicon: Matey, booty, Arghhh,savvy, buried treasure, plank, scoundrel, plunder etc. Say them in a low, gravelly voice. After all, you've been breathing in saltwater and are bound to be a little parched and raspy.
Don't bother with a real parrot. Visit your local taxidermist and see what they have. Anything with feathers will do.
When people ask you what the heck you're doing, tell them " Will Turner sent me..."
Instead of arguing with naysayers, request a parlay and insist that they "keep to the code."
Get into the habit of wearing an eyepatch. If at all possible, switch eyes every day so as to not strain one eye.
Develop a swagger and claim that you've got sea-legs, or because you've been drinking.
Go to the tanning salon often. Pirates have well-baked skin.
You need an earring, mate! For accuracy, go to a professional for goodness' sakes!
Read about real pirates and know trivia so when landlubbers ask you questions nobody doubts you are a true pirate.

[edit] Tips

People will doubt you if you are not 100% committed to the pirate lifestyle. See if you can borrow a boat from your local marina. At the very least, you can carry an oar about with you.
Note: the tip said "oar" and not "paddle" in the previous tip. The latter is a dead giveaway that you're not quite pirating.
The Pirates of the Caribbean series is a good place to pick up tips, though it is a modern interpretation. Especially watch scenes with Captain Jack Sparrow and Captain Hector Barbossa in them, as they are good for inspiration.
More authentic scenes can be seen in Roman Polanski's Pirates, 1986.
To really be taken seriously as a pirate, you need to encourage others (friends, family, your local shopkeeper) to become a pirate and join you on the high seas. Bank managers, lawyers, police officers, stock brokers, and teachers also make excellent pirates.

Pirates carry swords and sometimes pistols. However that is ILLEGAL on the mainland (in the UK, US laws on this vary by state), but toy guns and swords work great to get around the laws. It is also not recommended that you replace any limbs with wood or hooks.
Real piracy is illegal almost everywhere. It was, until very recently, theoretically punishable by death in the UK. BE CAREFUL!