Those purchase invoices you have paid during the month (actual invoice or receipt where is also stated the nature of the good/service and VAT information)
Write to restaurant receipt names of participants and what was the nature of the meeting (e.g. Client meeting, Representation, Meeting with new/potential client, Meeting with tax/marketing/etc. expert/consultant)
Save credit card related receipts separately so those are easy to find and match against credit card invoice (e.g. to own folder at DropBox or to eTasku application)