Whatever the case, make sure to invoice your clients consistently. You're more likely to get paid on time when your client receives invoices regularly on the first day of each month, than getting them sporadically.
Product or service details: Specify Description, Quantity, Rate, Amount and Subtotal. The more details you include on the invoice, the better. This way, your client will know exactly what they are paying for upon receiving the invoice. Make sure that your client can understand every item, so that as soon as they receive the invoice, they can pay you instead of asking questions and delaying the process.