Bulk e-mail folders and "false positives" for spam filters are the bain of an honest company's existence. It makes it so hard to get e-mail delivered to anyone.
The best way to do this is to ask your clients to "white list" the e-mail address that the invoices come from. They can do this with the help of the IT department or, for smaller companies, the hosting company will be able to help create rules in their e-mail software to eliminate this problem.
You might also want to look into an online invoicing system in which an actual invoice isn't delivered, but a link to an online invoice is.
Hope this helps,
Question added to topic Technology • August 14, 2008
How do we keep our invoices from landing in clients' bulk mail folder?
We send invoices to our clients once a month. The invoice often lands in their bulk mail folder. Is there anything we could change in our e-mail or setup to avoid that?