After you leave, keep in touch with these contacts by periodically meeting for lunch or breakfast. This way, you will continually be a candidate to receive outsourced projects. Get on the company's preferred vendor list. Using your contacts, you may be able to find the departments that are in charge of these lists and find out how you can be a preferred vendor. If you don't get on the company's preferred vendor list, then try subcontracting with a preferred vendor.
Many consultants have more work than they can possibly do themselves.
As a former employee of the company, you have an advantage over other consultants when applying to subcontract. You know who to go to in order to get things done. Try to get a number of different projects within the company and increase your contacts in a number of different departments. Develop a reputation once you have done some work for the company. Once you are in, whether on your own or through a subcontract, you increase dramatically your chances of making your former employer a good client.