All kidding aside, depending upon the scope of work, many contractors are willing to partner with homeowners to help keep costs down. This is especially true with my company when the customer is in the trades and wants help with areas they are neither licensed for nor experienced at performing.
My recommendation is to talk with the general contractor you intend to hire and see what you may be able to do in the project. So long as what you do does not affect their payment and is excluded from any warranty you expect from them, they may go along with it.
With that said, it's often better to let the contractors do what they do, while you do what you do for a living.
Emerald Estates Realty, Inc. Pro Home Improvements