Drawing is a skill that takes a lot of practice to gain, and develop. That said, you can optimize your learning by splitting it into lots of small, focused, skills, and limiting yourself to just learning one super focused skill at a time. that is:
take the time to highly focus your practice. 30m is not enough - take an hour at a time (and assume you'll waste 15m getting started/ready). take the hour every day - 6,7 days a week - at the same time in the day.
interweave exercises with your busy day. especially take the time to look at your surroundings whenever you can, and imagine drawing a section of it, focusing, in your mind, on the skill you're practicing that day. pay attention to any inspiration - and if you do feel inspired, act on it! even if it's just a 30 second doodle on a napkin at a bar - train your fingers to react to what your eye, and brain, sees.
In more detail, i think of drawing skills as sitting in three broad, incomplete, overlapping, categories - categories which can each be chased independently, or, if you have the time and ambition, in parallel.
This is the most generic, and most elusive skill to obtain - this is where most of your practice will go.
"eye" is, roughly, the ability to look at something, and be able to translate it into an abstraction which, when someone else looks at it, makes sense. There are lots of sub-skills, and hard to describe components - when practicing, though, focus on each of the following for a while, until you've made some progress, then move to the next:
- practice figuring out where edges are, and drawing them. A lot of this involves looking around and carefully finding the edges; when it comes to drawing, use a tool that you can't erase (say, a relatively thick pen or marker) and do quick, repeated, abstract sketches of interesting edges. say, the borders of a desk. the lines where shadow turns to light. if you have a lens or other optical distorter, put it in front of an edge, and draw that edge as it crosses into the distortion, stays inside, and then goes out of it. Many instructors advocate drawing the edges without looking at your drawing until you're done, or drawing things with a pen without lifting the pen off the paper.
Practice until you can reliably create understandable drawings of your subjects/scene
practice paying attention to shadows. use a bright light source to illuminate simple shapes at first (balls, boxes, bottles), and using a pencil or equivalent try to draw just the shadows that you're looking at (it's best to avoid explicitly drawing edges for this kind of practice).
study relationships, sizes, and depth. i find city scapes and buildings are good, early practice for learning core perspective concepts. practice drawing two simple objects, and moving them farther and farther apart for each other for successive sketches. learn to use two fingers to compare sizes for objects that you're drawing; practice using a view-finder (say, an empty frame) to help you draw an abstract of a scene.
practice drawing complex contours and compositions of basic shapes. if you're doing ok with spheres, for example, try drawing a loaf of bread. explore using textures, and shading, as a way to bring out interesting or unusual variations. find shapes in your surroundings that look different than you know them to be (for example, notice how the bottom of a tin can sometimes be best drawn as a straight line). draw them as they look, and then practice evoking the feeling of the shape as you know it - using textures, color, shading
"negative space"/"draw the emptiness". practice abstract drawing, where you focus on drawing the background (as a way to highlight the foreground or subject of your drawing). for example, put an apple on a table, and draw everything but the apple (shade it's shadow, draw the table, etc...). have someone sit on a couch for you, and draw the couch underneath them, leaving white space where the person is. this specific kind of drawing practice is most useful when interleaved with regular practice for other skills - say, do some negative space exercises once a week, in the middle of a few weeks of practicing a different skill.
each different material that you can use for drawing, will have specific properties and methods for generating particular looks, textures and feelings on paper and other mediums. there's too much to talk about specifically here - your first best bet is to sign up for a series of courses in drawing with the material you're most interested in - courses that are being taught by someone who you know is capable of drawing very well with the particular medium.
once you've gone past the very basic basics with a particular set of materials, and have spent some time practicing, the next step is to spend time with people who specialize in using that medium - find a setting where you can watch someone work (say, watch them draw with a pencil), or go to shows for art with the particular medium and ask the artist how she produced particularly interesting effects.
DOMAIN SPECIFIC SHORTCUTS
what are you trying to draw? many specific "domains" or "subject categories" have well established shortcuts. there are shortcuts, for example, for quickly getting the right proportions set for a human body, or for a cartoon character, or for drawing anime. if you've gained some general skills with drawing, you should be able to pick up these kinds of shortcuts by reading books outlining the various shortcuts for a given domain, and relatively little practice. Master artists can often intuit the shortcuts by looking at a final product.
Depending on where your skills, interests and available time intersect, you can spend your initial time in any of the above sections, and find immediate large strides. However, you're bound to fairly quickly reach the point where progress is going to be only made with regular, consistent, dedicated practice. Drawing is a mental and creative - but essentially physical skill.