In general, solutions to the pricipal-agent problem is by encouraging agents to act in the interest of principals by introducing incentives to align the goals of the two. Some of the ways in which this can be done are:
1) By ensuring that agents themselves have a vested interest in the outcome. For example, managers (agents) buy shares of the firm and not just draw a salary through options or giving shares as part of renumeration. It is in the managers interests to ensure that the share value rise so that they can benefit as well. This way shareholders (principals) can align both managers and their interests to increase the value of the firm.
2) By giving agents a shared interest in profit by tying some part of the interest directly to the outcome. For example, annual bonus that is linked to the previous year's profit of the company will mean that managers should take an interest in the wider outcome that benefits the principals.
3) By linking the agent's interest to the outcome. For example, linking manager's promotion to the profits of the whole company and not just the division.