Top things in no particular order:
1. Provide clean set of books and records. Depending on size of business, this may include the following financials:
General ledger, balance sheet, profit and loss, trial balance
2. Provide bank and cc statements so CPA can tie out your ending balance sheet for cash and cc liability.
3. For any real estate purchases/sales, provide closing documents. Your CPA may need to make adjusting journal entries after analyzing the HUD settlement statement.
4. For any rental properties, provide leases. Why? Because this has valuable data, such as security deposits, which need to be recorded as a liability on your balance sheet. Also advises CPA as to built in rent increases, who is responsible for taxes/insurance/etc., amongst other items.
5. Pay expenses out of the correct account. Do not commingle funds.
6. Call your CPA before entering a transaction. Tell them what you are contemplating. There may be something to consider from a tax perspective before proceeding.
7. Respond to open items and emails from your CPA promptly.
8. The list can go on, but it has been a long day for me. :)