The success of your email marketing campaign largely depends on the list you use for sending your messages. According to the CAN-SPAM act, email-marketing messages should be sent only to those who have permitted or opted to receive your mails. It is the most civilized way of marketing and the best way to avoid looking sloppy.
Here are some avenues that can help you to double the opt-ins for your email-marketing campaigns:

Your website:
Your website is the best channel to entice visitors to provide name and email addresses to send your offers. To use this tool to the replete:

Place a simple opt-in request (like newsletter sign-up) on all form pages possibly at the top fold.
Use motivational headlines and offers that capture attention.
Energize the visitor by offering links to articles, free downloads or samples that deal with target audience’s common interests. Provide the summary in such a way that the reader gets excited about it and downloads it immediately.
Outline your privacy policy committing not to pass-on personal details to any third party.
Trade Shows:
Trade shows have helped many businesses to capture data of highly targeted audience that could never be reached through any other channels. Proven strategies that compel visitors to opt-in for offers are:

Place signup forms or laptops facilitating the visitors to register.
Ensure that the business cards received from the opt-ins have email address. If not printed on the card, ask for it and obtain permission to send e-communcations.
Give out gifts to explicit opt-ins.
Display a board featuring the benefits of receiving your offers.
Your sales outlet:
Apart from being a point of sales center, your sales outlet can be put to the optimum use by taking advantage of the traffic to retain and generate more prospects.

Place signup forms in high traffic areas inside the outlet obtaining permission to send information on latest discounts and special offers.
An additional stream to get opt-ins is to offer gifts or other benefits to those who would give referrals of those interested in receiving your products or services.
Your sales and customer service personnel:
Since your sales and customer service personnel are the ones who have direct interaction with prospects and clients it is vital to use this resource to add opt-ins for email marketing.

Provide training to your sales and customer service personnel to explain prospects and clients the advantages of receiving your email offers.
Newspapers and magazines:
Newspapers and magazines have been very successful tools in obtaining subscriptions for advertisers. Most businesses report good return on investment by using this as an advertising media.

Advertising with newspapers and magazines related to your industry and having good circulation helps capture highly targeted prospect lists. This medium, already having laid groundwork to reach your target, would work in partnership with other advertising mediums for your success.
Opt-in list vendors:
The core focus of opt-in list vendors is making sure that all emails are pre-qualified to receive emails. Using such services offloads much of your important work and helps you focus on tasks that might require more of your attention.

Choosing a good vendor is vital to get a good return on investment
When you roll out your email campaigns by using the opt-in list you have thus secured, you can be assured of making huge profits. Rather than being intrusive, opt-in lists enable you to retain your old clients and win customer loyalty.