Growing your database
Electronic marketing mediums such as email and sms are extremely cost effective and return results on par with (or better than) other forms of marketing such as broadcast and print. However, the drawback on electronically marketing is that you need a database to send to.
The difference in costs associated with sending an electronic campaign to 1000 people as opposed to 10,000 people is negligible. So the key question is - what is the most effective way to grow my database?
Firstly, don't confuse quantity with quality. A campaign that you send to 1000 people that are aware of your product or company will return a far greater return on a campaign 99% of the time compared to 10,000 people that have never heard of you.
Secondly recognise that it takes time and effort to grow your database - so don't expect other people to hand over their databases to you (and you should never share yours). Also be wary of new people joining your organisation and "bringing their database with them".
You interact with customers and clients (both current and prospective) on a daily basis, but how often do you actually collect information from them so that you can market to them more effectively?
Does every person that comes through the door have the opportunity to join a "loyalty" club? Spotlight for example has on every checkout a simple A4 sized advertisement that invites customers to join their loyalty club. A great idea is to also offer an incentive to join (i.e. join our loyalty club / database we will give you 10% of your order right now).
Service industry (this is for all the lawyers, accountants and our web developer brethren who read this), your clients will nearly always email you. Do you collect this data in a meaningful way so that once a quarter you can market recent developments, news or promotions that they may be interested in? It's a great way to provide better customer service by educating and informing them. Clients may come to you only once or twice a year - so it is important to stay in touch with them on a regular basis.
Other marketing sources
Advertising via mediums such as print and broadcast (TV/ radio) are great ways to effectively brand your business. But imagine if you used this medium to not only generate sales, but grow your database.
You run a small B&B and you advertised in a local publication. In your advertisement, you could include a statement similar to "go to our website, signup to our eNewsletter and go into the draw to win a weekend away (drawn weekly)". Not only does this drives customers to your website, but you also grow your database (and if you monitor your statistics properly - you can also gauge the effectiveness of the campaign).
In summary, growing and maintaining a quality database takes time and effort. You should also treat those on your database with respect and aim to send them relevant and interesting information otherwise they may unsubscribe and will not want communications from you in the future.