To get results faster, use reference pictures as much as you want. Sometime, your pride may stop you, somehow telling you that you are “cheating”. It’s a mistake.
Tell yourself that you sketch in order to produce image and ideas. Sketching is not about memory performance (at the end, nobody care much :/). The most important is about how you are able to create, transform something existing to something new. If you can do that, awesome!
If you take the habit of drawing with reference pictures, your skills will improve faster, as you will feed your memory of forms (TIP 73).
Some pictures I took when I was chilling in Malaysia. A camera-phone is not only made for making selfies :). Take opportunities to take anything cool in pictures!
You can compare the sketches with the reference pictures – and try to find out which element I imported :).