Contact them before they call you, answer their questions quickly, and keep communications open. Early communication with creditors often avoids the need to deal with a collection agency. Usually, negotiating with the original creditor is easier than working with a collection agency to which the debt has been assigned. Once the account is turned over for collection, the original creditor loses almost all control over what would be an acceptable payment plan. Any good relationship from personal contact with the original creditor is lost.
Collection agencies are usually more aggressive and less willing to compromise.
In addition, since they frequently bring lawsuits against debtors to collect debts and are more familiar with the legal system, they are probably more willing to go to court than the original creditor, for whom the legal procedures may be confusing. Going to see the creditor in person is probably the least favorable to the debtor, with phone contact second. While something can be said for the value of the good faith shown by a personal contact, there are some real drawbacks. You have little control over the order of presentation. You might forget an important point, or your choice of words may not be the best.