Make a free plan that is very restrictive. You want people to be able to quickly assess whether or not the tool is useful to them. People are use to being able to trial software first (it's been done for years now). Allow them to try all the features, but not have any ability to use it for real production use. So let them accept 5 support requests, perhaps.

Alternatively, if you don't, you have to have great screenshots and screencasts. Chances are you will risk losing a lot of potential clients otherwise.