If the advocates of negative liberty look to protect at least some area in which an individual is free to do as she wishes, positive liberty advocates are much more ambitious - they look to enlarge this area of self-determined action as much as possible. They do this in two ways, the first being their inclusion of internal restraints in the conception of constraints to action. Rousseau, for instance, saw being a slave to one's desires or passions as the very opposite of being free.
Our desires are heterogamous, they come to us because of the environment we live in, or perhaps because of our upbringing. To give in to our desires, is for Rousseau, structurally similar to giving in to another's wishes. We have to consciously and rationally choose to fulfil our desires, that is, those wants that we see as really our own, and as reflective of our self. In his own words in 'The Social Contract', "the impulse of mere appetite is slavery, while obedience to a self-prescribed law is liberty." (Rousseau, 1967, p. 23)
Kant had a similar argument - how can one's freedom be evinced in actions that are the product of brute nature working through one by prompting desires which one blindly follows? Instead, to count as free, one must choose or select amongst one's desires according to some rational principal that one has oneself endorsed.
The second way of widening the domain of self-determined action in the conception of positive liberty is through democratic mechanisms of taking collective decisions. The emphasis is not so much on leaving as wide an area of one's life as possible untrammelled by laws, but since freedom is distinguished from license and defined as living under self-made laws, the emphasis is on ensuring that one has a voice in framing all the laws one lives under.
Coming back to Rousseau, the principle of liberty entails not only that we determine our wants, it also means that we frame the laws under which we live. Rousseau's advocacy of democracy is famous: there is no other form of government which is compatible with freedom. How can we be said to be self-determined unless we have a say in framing the rules that govern our actions. This is Rousseau's conception of civil freedom, in contrast to the moral freedom which prevents us from being a slave to our appetites.
Rousseau connected his conceptions of moral and civil freedom in the following manner: he saw the legislation framed collectively by the people when they keep the general interest in mind (by the general will) as a means of each individual being in control over his or her desires. In place of a person's own weak will, these laws, in the framing of which all participate, ensure that one leads a life chosen by oneself. Where the compulsion of the laws in the case of Hobbes, increased one's freedom by preventing others from interfering with one's action, in Rousseau, the interference of collectively made laws becomes a form of freedom.
After Rousseau, T.H. Green was an important advocate of positive liberty. In his 1881 essay, Green said: "We shall probably all agree that freedom, rightly understood, is the greatest of blessings; that its attainment is the true end of all our effort as citizens. But when we thus speak of freedom, we should consider carefully what we mean by it. We do not mean merely freedom from restraint or compulsion. We do riot mean merely freedom to do as we like irrespectively of what it is we like. We do not mean a freedom that can be enjoyed by one man or one set of men as the cost of a loss of freedom to others. When we speak of freedom...we mean a positive power or capacity of doing or enjoying something worth doing or enjoying, and that, too, something that we do or enjoy in common with others...the ideal of true freedom is the maximum of power for all members of human society alike to make the best of themselves..." (Green, 'Liberal Legislation and Freedom of Contract', 1881, pp. 199-200)
Both J.S. Mill and T.H. Green concurred in seeing the value of liberty in allowing individuals 'to make the best of themselves'; yet they disagreed about the definition of liberty. This is the point that bears thinking about - why is it important for individuals to be free? Why does agreement on the value of liberty still lead to differences on its meaning?)