One technique I use to develop this faculty is focusing on a particular point in space for an extended period. This helps you to become hyper-aware of your surroundings. We tend to focus on physical things: chair, wall, etc. Trying to focus on a space in the invisible no thing improves your focus, allowing you to concentrate on things not readily picked up by the physical senses. You begin to gain a feeling that there's something out there, and this sensation is not picked up by the 5 senses. Kind of freaky, and cool too.
Do you practice developing your Sixth Sense?