What are the most effective techniques used in collecting requirements as a business analyst?

The most effective technique is directly interviewing the end users and subject matter experts.  Ideally, you want to take their findings, map the business processes (you can use graphical forms such as BPMN, and you can use SIPOC charts to add details of inputs, outputs, customers, suppliers, and a written process description).  Once you have that, you need to take this documentation back to your end users and make sure they tell you if you've understood them.  This process is iterative, meaning you probably need to repeat it several times until the business processes are clearly understood, and all exceptions to the normal process flow are captured.