Time
LA BCIS office opens at 7 AM. It's better to go before 6 AM.
Which line?
You will find two lines there. Do not stand on the long long line which starts from the left side of the building. Stand on the shorter line which for only I-485 approval. It's starts from front of the building.
Parking
Take left turn from Los Angeles St to Temple St. You will find a parking lot at right side (on Temple St). That's for $6.00, the cheapest parking you can get there. There are some other parkings at Los Angeles St too.
What documents?
For my case, the INS person was pretty warm and funny. He greeted me with "welcome to INS" and while leaving "thanks you for shopping at INS". He did not ask for any document - except my passport and EAD card. But for one of my colleagues case, they asked for recent pay stubs. Here, the list of documents you should carry. It's an exhuastive list, but it's safer keep them handy. I marked important ones in bold.
1. Passport + I94 (INS will take back I94)
2. EAD card (INS will take back it)
3. Advance Parole (INS will take back them)
4. Marriage Certificate (if married)
5. Birth Certificates or Affidavits
6. Tax return (if you did not file 2002 tax returns yet, take 2001 tax return and 2002 W-2 forms)
7. Recent pay stubs
8. Bank statements of joint account or apartment lease agreement where both names are present (if any. It's a proof that marriage is not fraud)
9. Health cards
10. Drivers Licenses
11. Social security cards
If you are single, you can strike out some of items. You know what are those.
Anything more?
Office opens at 7 AM. That means you will be able to enter into office premise after that. After entering the building you have to go (read "run") 2nd floor and submit the your approval notice to a clerk sitting at the door. Faster you reach to clerk, faster you will get out. It's big building. Do not be confused and waste time. From the front gate, go to right side and catch the elevator. From elevator, at 2nd floor, follow the corridor at right.
BTW, if you do not live in Santa Ana jurisdiction area it's not good idea to go there (as sks2003 suggested). They may turn you back (though it depends on the clerk/officer who is handing). It's better to go to the office suggested in your approval letter.