Here are some commands I had to use this week. For exporting users Mailbox Items to PST (and the ones that were too big to Yearly PST)
# Create PST of users mailbox + By Date - Note - Location must be a UNC Path
New-MailboxExportRequest -ContentFilter {(Received -lt '03/1/2013') -and (Received -gt '03/01/2012')} -Mailbox "User" -FilePath "\\Location\pst\User.pst"
# See mailbox export queue
Get-MailboxExportRequest
# why exports are failing with details
Get-MailboxExportRequest -status failed | Get-MailboxExportRequestStatistics -IncludeReport | Format-List > c:\temp\report.txt
# Clear out completed job queue
Get-MailboxExport Request -Status Completed | Remove-MailboxExportRequest​