Just as the photography and the production of a job is important so is the follow up. Specifically meaning invoices.
Invoices can be annoying and a daunting task. As far as agencies go, I have found that a project will usually get a 50% advance (sometimes 75%). As an art buyer closing the job is super important and time consuming. I love when photographers/reps send a detailed and comprehensive invoice. Line item by line item with the receipts to back it up. You always have to have the proper receipts attached otherwise the accounts payable people will not always pay them. The backup information is key to the bottom line. Art buyers and accounts payable will go through them (usually) with a fine tooth comb to make sure everything matches up perfectly. I have found both sides benefit... I once had a grocery receipt attached as misc. expenses (had that removed) and I have also found that the rep company miscalculated a cost and had to have additional money added into the bottom line.
Have a detailed and easy to read format and system for your invoice. The more concise your invoice, the easier it is for the art buyer to go through and phone in with any questions and the faster it gets to the accounts payable people.
Tips I have learned:
Lost your receipt: put everything on a credit card. If you lose the receipt you can send the credit card statement with the missing receipt highlighted.
Send back all non-returnable props, wardrobe, etc., no matter how crazy the item. The clients have paid for it and they'll want it back or to at least have the option of keeping it or donating for a tax write-off.
Get overages signed off on while on set with the art buyer/producer and client. I have never experienced a client not paying what is owed but they hate being surprised with overages after the fact. I will always write out what the overage is and have the account manager and client sign off before it is incurred. But it is super important to discuss the overage before it's incurred.
Charge for mileage if you're driving around from location to location. Especially with gas prices the way they are you should be charging for mileage (I think the current rate is .485 per mile).
Always state when the payment is due (i.e. 30 days from invoice date is usually pretty standard) and if past due the percentage of interest that will be charged that way there are no surprises. I always try to process the invoices as quickly as possible and submit them but agencies and clients have their own billing cycles which may not match these dates. I have also found that once it leaves my hands I'm not aware of the status until the photographer/rep calls or emails and lets me know they haven't received payment (usually it's 45 days past at that point). Always keep up with your dates and payment, it never hurts to send a follow up or checking on payment status email to the art buyer/producer
Make the invoicing as smooth as possible (just like the shoot and the beautiful photos you produced) and you'll have repeat business.