What I don't understand is who or what is going to ever be able to change the constitution (or whatever) so that it works automatically to incentivise those in power to work for the general good? If there was a body or person that had the power to change constitutions in such a way wouldn't that body also be more likely to work for its own good unless there was in turn some incentive for it to change constitutions in this way?
OTM -- and this is basically the most important debate in development / economics today, IMO.
Rich, i think the last couple of paragraphs at the end of this post should shed some light on your questions. But first I just want to try and explain some of the basic concepts of the institutional approach to political economy to try and make it all a bit clearer (or perhaps just complicate matters further

).
So what is that is that creates the incentives and constraints which govern human interaction? The answer (at least according to this model of political economy) is INSTITUTIONS.
What are institutions? The definition provided by Douglass North - one of the founding fathers of New Institutional Economics - is:
Institutions are the rules of the game in a society or, more formally, are the humanly devised constraints that shape human interaction. In consequence they structure incentives in human exchange, whether political, social or economic. Institutional change shapes the way societies evolve through time and hence is the key to understanding economic change.
North differentiates between what he terms ‘formal’ and ‘informal’ institutions; the former tend to be explicit rules embodied in law or constitution, while the latter tend to be implicit codes of conduct and norms of behaviour embodied in custom and tradition.
Furthermore North draws a vital distinction between institutions as ‘the rules’ and organizations defined as ‘groups of individuals bound together by some common purpose to achieve objectives.’ Thus bodies which are commonly referred to as institutions such as the World Bank and International Monetary Fund (IMF) are, by this definition, considered to be organizations, as are States, firms and all manner of social bodies.
Laws, norms and codes of conduct evolve to regularise patterns of human interaction in order to promote stability and reduce the uncertainty of outcomes. The functionality of any given institutional arrangement requires a set of enforcement mechanisms – a system of formal and informal punishments – which ensure that actors, on the whole, adhere to the rules.
Such constraints on behaviour limit the opportunity set available to actors and define the relative rewards they can expect for exploiting these opportunities. These rewards, which North terms the ‘pay-off structure’, in turn affects the types of organizations which evolve within the society in question. Particularly important in this regard are the formal economic institutions governing property rights and the enforcement of contracts. If these formal institutions are weak and as a consequence the greatest pay-offs are made to rent-seeking behaviour, then organizations are likely to cultivate such opportunities (for instance expropriation of property, exclusive monopoly privileges, and the like). By contrast, if property rights and contract enforcement are well protected then the highest returns will go to organizations engaging in productive activities. If the latter is the case then ‘we will expect organizations to devote resources to investing in skill and knowledge that will increase productivity.’ Thus, it becomes clear that the institutional structure of a society is a key determinant in its economic performance. The incentives offered by this structure influence both the way in which resources are distributed and the way in which they are used.
What is missing from this analysis is an account of the factors which underlie continuity and change within the institutional structure of any given society. Developing an understanding of this requires an examination of the complex interplay between beliefs and power in shaping institutions. The cultural beliefs of a society - its norms, values and ideas governing interaction - are transmitted from one generation to the next as a form of acquired knowledge. For this reason, North maintains that
cultural heritage – or ‘path dependency’ - explains the relative continuity of societies through time and
acts as a powerful impediment to radical change. Such continuity occurs because informal institutions provide a historically accumulated framework which is not typically amenable to deliberate human manipulation. Furthermore, since organizations (which include the State apparatus don't forget) emerge to take advantage of the opportunity set provided by the institutional framework, they have a
vested interest in preventing any alteration to the framework that threatens their survival.
Therefore, it must be recognised that institutional change is a two-tiered process. Changes in the formal rules of a society can be achieved relatively quickly and easily through decisions taken by those who hold political power. However, such changes are likely to occur only if they are perceived by the existing elites to be incentive-compatible with their interests. By contrast, informal constraints, due to their embedded nature in custom and tradition, cannot be altered in the same top-down manner. Therefore, as a rule, societies change by building on and modifying their existing institutional structures rather than radically reversing their direction overnight; and thus ‘the huge variation in the world today in both economic and political outcomes is the result of long-run historical processes.’