It is not emphasized enough that everyday sketching is absolutely important

It is like trying to get fit in the gym, you need important muscle memory to develop this skill. 

Practice the variety of sketching techniques, you can do one from each set every day Categorize them into 5-6 categories, and by the end of each week, you must ideally have about 35-40 sketches. 

The number is important while practising. Do free-hand illustrations and anatomical drawings as many times as possible.