1. Run your export when your database is not so bussy, i.e. when there is low ammount of changes to the database by other sessions.
2. If you are using CONSISTENT=Y option in your export, reconsider again if you realy need tables to be exported in consistent mode in relation to each other. With CONSISTENT=N you have much less cance to get ORA-1555
3. Make your rollback segments bigger and/or make more of them available
ASCII a stupid question, get a stupid ANSI
24 hours in a day .... 24 beer in a case .... coincidence?