As mentioned above, offer consulting services and accept corporate purchase orders. Be able to give and receive PO numbers. This is the language the bean counters speak, and sprinkles magic pixie dust on your invoices, ensuring you get paid.

One really big downside to offering this yourself or providing it as a service: contracts. You can try avoid paying a lawyer and use a standard contract. The engineering manager would be cool with that since he just wants to support a project but all bets are off when it hits legal. A big enough company and you can get into a seemingly endless revision back-and-forth that can last weeks. No big deal for them and other big companies they work with since they have salaried lawyers on staff but the cost is something to consider for everyone else.

Hiring a lawyer is important if you are going to be contracting with a large enough company though. The money you save and the time you spend as a result of having a competent lawyer draft, negotiate, and proof your contracts pays for itself in the long run when, inevitably, something goes wrong on a project.