It helps in concentration if you just do one thing at a time giving it your complete undivided attention.

Multitasking (doing many things simultaneously) will make you distracted and it is an ineffective way of doing things contrary to what many people believe. Do one thing at a time in order to develop concentration. If you are trying to think of giving presentation at office, or studying for your exam, for the next 30 minutes or so, just think of that alone. Focus your mind on the details of the job and how you plan to do it.