It sounds like you might want to do two things.  First, adding a note on your invoices about your payment terms.  I do this myself on my invoices.  Just open up an invoice, click Customize, select the Footer tab, check the Long Text Disclaimer box, and enter what you’d like shown at the bottom of your invoices.  I use the following for mine:

Payment is due upon receipt of invoice.  After 30 days, interest will start accruing at 12% annually.

In terms of friendly reminders, QuickBooks offers statements (Customers > Create Statements).  We invoice weekly and send statements on the last day of the month only to customers with unpaid invoices by selecting All Open Transactions as of Statement Date.  The statements look quite a bit like invoices, especially if you elect to show Invoice Item Details on Statements and Print Due Date on Transactions, like we do.  You can also customize your statement by clicking the Customize button.  Once you’re finished, make sure to Preview the statements before either printing or emailing them (we email to everyone and print/mail only those that are over 30 days late).

For your emails, you can customize the default email by going to Edit > Preferences > Company Preferences > Send Forms and selecting Statement in the Change Default For box.  While you’re there, you can also change the default for any other forms you send by email including your invoices.