Check formatting, spelling & grammar and ensure invoices are correct before sending. Unprofessional invoices can affect your brand image and even prevent you from getting paid!
Create an email address specifically for dealing with invoicing and accounting. For example, set up accounts@yourcompany.com and send all your invoices from that email. This will create a stronger impact on the buyer when they are asked to be pay.
Set crystal-clear terms on the invoice so that you get paid on time, and include late fees associated with late payments on the invoice.