using require() to prevent fail without throwing an exception.
restricting other contract's access to state variables by explicitly define private variables, and also restricting function access so that only the contract owner or a specific addresses are permitted to execute functions.
Using events to monitor contracts activities.
implementing the mortal design pattern means including the ability to destroy the contract and remove it from the blockchain using the selfdestruct keyword.
using a withdraw function which users could call it and get their funds instead of sending funds to the users by the contract. It protects against re-entrancy and denial of service attacks
it allows contract functionality to be stopped. This would be desirable in situations where there is a live contract where a bug has been detected. Freezing the contract would be beneficial for reducing harm before a fix can be implemented. In this case freezing the contract prevents employers add new projects and freelancers pick the open projects while still allowing employers to withdraw their funds.
Using the Master/Slave proxy design pattern to make the smart contract upgradable, by implementing a Proxy contract and a MasterSlave interface which is common between the proxy and the Freelancing contract.