I don't think you can paint all opt-in lists with the same brush. I would take several steps.
 

1. Work with list brokers who have a good recommendation in the direct marketing community. List companies can't stay in business with disgruntled customers or with lists that don't produce results. They help you find the lists targeted best for your need and are paid by the list owner.
2. Verify how the names were obtained. Ask to see the web page(s) where the names are solicited and see how much permission the person granted for additional e-mailings.
3. Test a small portion of the list to determine results and then do a full campaign on the lists that work best for you.

List brokers never give you the e-mail addresses but perform the mailing for you, often with better response tracking systems than you can afford on your own. Expect to pay US 15 to 30 cents per name and e-mail address.

Your Own Opt-In-List: (The best of the best.)

There are several approaches you can use to increase your company's house opt-in e-mail list. Some should be used in combination with each other for maximum effect.
 

1. Offer an inducement to sign up for the list. It's best to offer something that you don't have to mail or ship, such as premium information that can be downloaded at minimal cost. Consider using a sweepstakes or contest approach, or offer a discount coupon that your clients would value. Make sure, however, that your inducement will attract the kinds of sign-ups that will make good clients for your company's goods or services.
2. Contact the owner of a complementary list and offer to swap ads that allow each of you to grow your list from the other's subscribers.

Contract with an organization that will find new list members for your company on a per-subscriber basis (15 to 25 cents per subscriber). For smaller needs try www.WorldWideLists.com;for larger needs contact SparkList (www.sparklist.com/services/paypersub/ ).