It often depends on the project, whether or not it is a government or private, length of the project, dollar value, ect. If it is government there are some avenues that you can take, especially if you are a small business or minority contractor. these will require research and generally a lot of paper work, but can be very worth while.
We have negotiated with clients on particularly long projects to receive a percentage of the retainage upon substantial completion of a given line item; however it is usually done during contract negotiation before the project startup.
Typically retainage is in lieu of a bond on private projects (such as the $8m I am presently working on has a 10% retainage. (that’s a lot of dough over 2 years)) to cover any subs or work related issues. If you can get them to write off on a particular phase of the work and show that your subs have been paid it will be up to them to allow or disallow releasing a portion of the retainage (keep in mind the client will not want to give up the retainage that he is either collecting interest on or using to supplement he cash flow). You will have to show some advantage to him to entice him into releasing it, meabe in the form of some early payment reductions, such as a percentage off the bill for 10/20/30 day payment terms, ahead of schedule completion of certain line items, ect.
Like I said it is far better to get these things out of the way during contract negotiations so it is up front.