out of all my drawing courses i have taken in the past 5 yrs i absoultly learned the most and gained the most knwoledge about drawing in Life Drawing courses. this helps with seeing shapes of objects not what you percieve something to look like. and makes you concentrate on really studying the thing your drawing and has greatly improved my art. so definately sign up for a few life drawing classes. also look up blind contour drawings this helps a lot with studying the shape and form of the object your drawing.

the best way to improve your drawing is to tak a life drawing class, even if you arnt that interested in anatomy or form , you can do still life classes with objects . anyway my point is the most important thing you lear is not drawing but seeing , or looking – your drawing improves with practice and your skills greatly increase when you train your ability to break object down into light and shadow . later then comes the skill to draw and render from mind, based of your stored knowledge of how different surfaces react to light ..
as aposed to copying photos or other drawings , from life object you really coach your ability to decipher where shadow and highlight goes for yourself – it will also bring out your own style – if you dont want to do a class than just practice drawing different objects in different lighting also diversify your medium , paint charcoal pen pencil – this stops dependence on the medium to carry the style and will make it much easier for you to pick up anything and draw well – stick in sand and most importantly

another good exercise especially if you want to do tattooing is draw lots in pen – or dont erase , just redraw on a new sheet refining your ability to control the first line
exercises like drawing without taking the pen off the paper and not looking away from the object to see the paper (can look a mess) helps the communication between what you see and what your hand draws .

keep at it mate