Filing.... well I tried a few systems over the years, but here's what I've found works best for me and the accountant.
I have +10 properties which I self-manage. Each property has its own A4 file for the financial year. Each file then has dividers inserted:
SUMMARY - important info about the property - date of purchase, price etc;
INVOICES - bills/accounts
DEPRECIATION REPORT
LEASE doc
BANK statements
CORRESPONDENCE
OTHER - photo's, marketing ads, reno info - paint colours, products used; trades contacts
I also construct one spreadsheet per property based on the categories of the Tax return, and prior to tax time, summarise all expenses & income etc for each IP.
I meet my accountant just after the financial year and give him a 'Bankers Box' that contains all the files, and a downloaded e-copy of the spreasheets onto a thumbdrive.
On a weekly basis, I simply check my mailbox every day and anything that can be filed (or binned) immediately is. Any accounts and invoices I simply stick (in date order of payment) into a manila folder. Once a week, I jump onto the internet and pay the bills, and/or write cheques & then file invoices into the correct A4file. I use a manual diary for recording renewals of leases or any bank/refinancing schedules etc.
I've used this method for a few years now and it works for me (...and the acocuntant loves it). I just got audited by the ATO this year and the filing system worked a treat!
I know it sounds like a lot of work, but it only takes me about 1 hour each Sunday to keep everything in order for all the properties.
Good luck with whatever system you choose to go with