The format I use on invoices is:

1st Page: Breakdown of Charges:
Hours Charged
Callout Charges
Materials Charges
Total Charges (ex-tax, tax, inc-tax)
Discount provided

1st half of 2nd Page: More detailed chart of charges:
Hours Worked, Hours Charged, Deduction (if applicable)
Callouts Made, Callouts Charged, Deduction (if applicable)
Materials Charges
Total Deductions (if applicable)
Total Charges:
Without deductions
With deductions
Ex-tax, Tax, Inc-tax

2nd half of 2nd Page:
Outline of Materials Charged
Outline of Deductions Applied
Reason for Activity
Service Sessions

3rd Page onwards:
Outline of services performed (even if a little technical, helpful for future reference)
Outstanding Issues
Recommendations for future consideration

I try to be as transparent as possible. I sometimes worry if I overdo it. Some customers seem to respond well to it. Sometimes it backfires with some customers who go through the outline of services performed with a fine tooth comb and find something to pick on. It seems that it's difficult to please everyone. I figure that some customers don't pay much attention to the outline of services performed, but at least it's there for them, and they can see that I've done a lot even if they don't understand it.

Overall, I figure as much transparency as possible is best.

Mark Reynolds
www.markreynolds.com.au