I’ve read that if you are dealing with a collection agency, you should start negotiating at 25% of your debt. If you are dealing with the original creditor it might be different. Just make sure that whatever percentage you agree on, it counts as full payment of your debt. Otherwise they might try to collect the balance later on.
I found the following articles very helpful:
– Pay for delete: http://www.creditinfocenter.com/debt/pay-for-delete.shtml
– Settle a debt with a collection agency: http://www.creditinfocenter.com/debt/settle_debts.shtml
– Negotiating with original creditors: http://www.creditinfocenter.com/debt/originalcreditor_debtsettlement.shtml