Private student loans do not have the same gaurenteed options for getting out of default as federal loans. Basically, you need to figure out what you can do (what type of lump sum and/or monthly payment can you afford) and then contact the lender to see what they will accept. You could check out the Rebuilding Your Credit forums and look for folks who dealt with charged off installment debt for some ideas.
How old is the debt? You will also want to know if it is outside the statute of limitations - once that happens, you can successfully defend against a lawsuit over the debt, and most lenders are far more likely to take a reduced settlement because they aren't able to collect the money through a judgment.
