How can one improve Import/ Export performance?

EXPORT:
1) Set the BUFFER parameter to a high value (e.g. 2Mb -- entered as an integer "2000000")
2) Set the RECORDLENGTH parameter to a high value (e.g. 64Kb -- entered as an integer "64000")
3) Use DIRECT=yes (direct mode export)
4) Stop unnecessary applications to free-up resources for your job.
5) If you run multiple export sessions, ensure they write to different physical disks.
6) DO NOT export to an NFS mounted filesystem. It will take forever.

IMPORT:
1) Create an indexfile so that you can create indexes AFTER you have imported data. Do this by setting INDEXFILE to a filename and then import. No data will be imported but a file containing index definitions will be created. You must edit this file afterwards and supply the passwords for the schemas on all CONNECT statements.
2) Place the file to be imported on a separate physical disk from the oracle data files
3) Increase DB_CACHE_SIZE (DB_BLOCK_BUFFERS prior to 9i) considerably in the init$SID.ora file
4) Set the LOG_BUFFER to a big value and restart oracle.
5) Stop redo log archiving if it is running (ALTER DATABASE NOARCHIVELOG;)
6) Create a BIG tablespace with a BIG rollback segment inside. Set all other rollback segments offline (except the SYSTEM rollback segment of course). The rollback segment must be as big as your biggest table (I think?)7) Use COMMIT=N in the import parameter file if you can afford it.
8) Use STATISTICS=NONE in the import parameter file to avoid time consuming to import the statistics
9) Remember to run the indexfile previously created.

Common Import/ Export Problems

ORA-00001: Unique constraint (...) violated
You are importing duplicate rows. Use IGNORE=YES to skip tables that already exist (imp will give an error if the object is re-created).

ORA-01555: Snapshot too old
Ask your users to STOP working while you are exporting or try using parameter CONSISTENT=NO

ORA-01562: Failed to extend rollback segment
Create bigger rollback segments or set parameter COMMIT=Y while importing

IMP-00015: Statement failed ... object already exists...
Use the IGNORE=Y import parameter to ignore these errors, but be careful as you might end up with duplicate rows.