1. Debt validation.
This only works with collection agencies and only after they have sent you their initial letter informing you of their attempt to collect. Send them a letter asking for a verification of the debt (how do you know they are not a scam?) and proof that they are licensed to collect in your state (how do you know they are who they say they are?). Quote the relevant portion of the Fair Debt Collection Practices Act (FDCPA): Section 809(b). If they don't respond within 30 days,

2. Pay for Delete.
If you have sufficient cash to pay off the entire amount you owe, you can try asking for the collections agency to delete the entire account (as if it never existed) in exchange for paying everything you owe. A lot of collection agencies may not be able to actually do this, or may refuse. A lot depends on luck and your negotiating ability. If you have enough to pay off the whole account, and you don't ask desperate, it can't hurt to try. You need to keep the haggling upper hand, though. Make them think they'll get either the whole amount or nothing.

3. Goodwill.
This can be surprisingly effective if you are tenacious enough about it. You basically write a letter describing your financial situation (not TOO much of a sob story, though) and what you had to go through after you've already paid, and asking them to delete all trace of the account. You may be better off trying this with the original creditor than a collections agency, and it doesn't always work, but it can be worth the 41 cents to give it a go.