I've been taking notes along my training and one of the things that has helped me the most on making my senses keener, as JMG says, is being very attentive and still in nature. In particular around the hours the yogis call Sandhya Kalas --sunrise, noon, sunset and midnight--I found there are very noticeable things changing that the senses perceive and can be a nice experience to go through them in a meditative state.