Billing before you start the project
The first viable option is to bill your design client before you even get started on the project. Some designers would argue this is the best option because you incur no risk of getting paid. Contrarily, it might be hard to convince your client to take all the risk, trusting that you will complete the project as promised.
That means there are two real options for billing before you start the project. Partial payment and full payment.
Partially
Asking for a partial payment before you start on the project is a widely accepted way of doing things in the design community.
It creates a commitment from both parties: the designer feels obligated to complete the work for which he has been paid in advance and the client feels obligated to finish out the project so as to not have wasted his down payment.
In Full
Another option is to request payment for the entire project before beginning the work. While it’s hard to convince a client to incur all the costs prior to completion, this method can come in handy if you need to by any new stock footage or photography, or need to hire any photographers, typographers, printers, etc.
Billing during the project
If billing before the project doesn’t sound like a great option for your design business, maybe you’d like to consider billing during the project. I would suggest one of two options (but would also love to hear your suggestions) when billing during the project: billing at time-based intervals and billing at project-based intervals.
Time-based intervals
This option involves the designer working on the project and billing every few days or weeks. I personally bill every two weeks for work completed during that time. It keeps both parties on task and encourages work to move forward because, if two weeks go by and nothing gets done, no one gets paid.
Project-based intervals
The other option is to bill clients at predetermined intervals during the design process. I’ve also tried this method and it worked nicely because the quicker I got any given portion of the project done, the more quickly I got paid for my work.
For example, if you’re working on a web site, perhaps you bill after working out a site plan, then after drafting up wireframes, then after a preliminary design, and so on.
You get the idea.
Billing after the project is completed
Lastly, you can send your client a bill after the project is completed. Converse to the option of demanding full payment upfront, this option places all the risk on you, the designer. If, for any reason, your client decides he doesn’t want to pay for your services, there’s nothing obligating him to (except that, of course, you should make it clear in your contract when payment is due).