If I were to start completely over and had to build my list from scratch I would do a few things.
1. I would create a few squeeze pages and offer a free report and do some forum marketing. If you are consistent in your efforts (which should only take 10-20 minutes per day if you are hitting up a few forums each day) you can probably get 25-50 sign ups per week. It doesn't sound like a lot but after a few months you have 1,000+ subscribers. This is also good for building your "brand"
2. WSOs is a fantastic way to build a list quickly. I am doing a report right now for you guys here at JVNotifyPro that will detail all kinds of good stuff about WSOs so be on the lookout for that. Either doing 100% commission up front, doing free WSOs, or war room reports, you can get a few thousand people on your list relatively quickly...and the quality is normally pretty good. Again, this is also great for brand building.
3. Also as mentioned above, launch download page gifts (through opt in) is a good way to build your brand, build a relationship with the product owner, and build your list.
One thing to keep in mind is that building a list is only half the battle. Marketing to your list is just as important. A good marketer can make more money from a 2,000 subscriber list than a bad marketer to a 100,000 list. Provide value, establish rapport, and legitimately want to help your list succeed. Sounds simple but makes all the difference in the world. I want each and every person on my subscriber list to hit six figures online, I truly do. This translates into me providing a lot of value and in return, I do quite well. It's a win-win-win.
"A good marketer can make more money from a 2,000 subscriber list than a bad marketer to a 100,000 list." ... I've seen a merchant with 9000 (approx) on her list make $17,000 for the exact same offer that a "big name" marketer with 60,000 on his list made $800 for and (this one's fun), another "big name" marketer who claimed 125,000 on their list and made $2200 with.
9000 = $17,000
60,000 = $800
125,000 = $2200
These are real numbers (well, the revenues are... the list size is just from conversations of what those affiliates stated that they mailed to).
So I agree and I often harp on the fact that the database size isn't relevant. It's the quality of the relationship with those leads.
As Ken just suggested, please think bigger. Many will stop at "how do I get a lot of people onto my list" and, in reality, the money is in "how do I get people on my list to know me, love me and trust me".
And while this is somewhat self-serving, I have some conversion info in my I QUIT report (that recently went live)... it's direct download / no opt-in so no worries... but I recommend read pages 47 to 57. :-D