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The nature of the program will determine the assets you will need. Do you need custom sounds? Artwork? Content? All of these questions should be answered before you release your program. If you need a lot of assets, but don't have the manpower or talent to create them yourself, you can consider outsourcing asset creation. There are countless freelancers out there that may be willing to work on your project. Make sure that they do not interfere with the functionality of your program, and that there is nothing superfluous. Adding assets usually occurs int eh final stages of the programming cycle, unless the assets are integral to the program itself. This is most often the case in video game programming.
Determine what you'll need. Consider outsourcing. Implement your assets.