From working with freemium and looking into best practice examples, I have isolated a few characteristics of companies prone to freemium success.
Quality free product that people want
The most important condition for creating a successful freemium model is that you have a great product that people want. It will be the engine that drives your freemium based business. If this free product does not have inherent value your freemium will not get off the ground.
So instead of giving away a sample track of the music, let people download the whole album. Instead of letting people take a look at your worksheets, let them have all your tools for free.
The free product can be duplicated digitally
Only a small percentage of the free users will usually buy something. In order for this to make financial sense, the expense of distributing the free product should be very minimal. Digital duplication ensures virtually no cost for copying and distribution.
If you want to distribute 1 million pieces of something, $1 as unit price is quite a substantial amount.
Freemium depends on generating attention with the free product; then to sell premium products or services to some of the free users. In most cases only a small % of the free users will buy something. This is is no problem, as long as it is a small percentage of a large number.