Charge From Day One
if you plan to one day sell you product, you'd better make it day one
That's the conclusion I came to after a brief chat with Eric Ries.
Even if you offer a free version, you need to have some sort of premium, paid version. You need to get your users accustomed to the idea that your product, in its full form, costs money.
Last.fm had a lot of trouble with this. At first, you could listen to online radio for free and you could pay for a subscription to get more control over the radio, get more tracks, etc. Most people used the free version because, well, people don't want to pay. Then, the people at last.fm decided that they need more revenue, so the lowered the subscription cost, and made it so that you can only listen to radio stations if you have a subscription. At first, this almost killed them. They pulled some voodoo quickly and now they're still around.
Mr. Unfollowr is a twitter bot that sends you a direct message when somebody unfollows you. It's a great tool and about 37k people are using it. Obviously, everything was free in the beginning. However, the developer is now trying to earn some cash off his nice invention. The thing goes like this: you make a one-time donation ( even $5 ) to the dev and you get DMs the instant somebody unfollows you. Well, almost instantly. Those who don't make a donation, they have to wait, sometimes even a couple of days, until they get notified. I think that's pretty fair.
The problem is that only 15 people have made a donation so far, and only $96 have been raised. That's a very small conversion rate ( 0.04054% ).
And that is because people were used to getting it for free.
Also, I think the fact that there are alternatives out there ( to both services ) makes a big difference ( in both cases ).
You need to make your users aware that you're going to charge!