Seriously, there is only one way to learn and that's practice.
And don't stop working on an image until you think it's perfect, a lot of people give up thinking 'that'll do', keep going! This took me about 16 hours to SKETCH - not even to ink and colour!
Pulse cover by ~Jesse-Yuki on deviantART Lol! Now I can do something similar in maybe 5 hours. Practice! ^_^
But for technical drawing, I only have one tip and it's the only one you need IMO. When you're trying to draw something, look at it! Trying to draw a face? Look at a real (or photograph of a) face. Shoes? Look at shoes. etc. etc. Doing this will help you no end and it will sharpen your observational skills by drawing what is actually there and not what you *think* *should* be there.