The fume hood is a ventilated workbench that protects you from hazardous fumes and shields you from blasts. The fumes are drawn up and any hazardous chemicals are filtered out. The fume hood is only doing its job if a good air flow is guaranteed.
You need to ensure following points when working with a fume hood:
Make sure the air flows - if the thing is off it won't protect you! Sometimes it's not obvious; stick a small piece of paper into the hood and you will immediately see the air flow.
Work with the sash down. There is usually a marking on the side of the slider the tells you how far up you can move the sash without compromising the air flow. Get used to working with that piece of glass in front of your nose. If something blows up inside the hood it will protect your pretty face!
Place your reagents in the back of the hood. The closer to the opening the more likely that fumes can be blown out by your hand movements.
Remove unnecessary objects, each one of them is an obstacle to the airflow. If you don't need it, remove it!
Figure 1. Pull the sash down and place that fuming beaker towards the back!
You are safe to work in the hood, check this out: