2. Invest in a daylight spectrum lamp - If you're doing full-colour traditional work and you don't have a strong source of natural light this is very important. Standard light bulbs have a warm tone and halogen bulbs have a cool tone, both of which distort your perception of colour. A daylight spectrum lamp simulates bright, natural light which not only gives you a better grasp of colour but stops your eyes getting tired out. I haven't stopped using mine since I bought it, and while they are expensive you can usually pick them up cheap used!
3. Work in high resolution - A few years ago a friend of mine berated me for working in ACTUAL SIZE on digital pieces when I should be working in high resolution. Not only is it far easier to get detail and depth in, if you ever plan on getting work printed you can forget about it unless it's created in high resolution. That is, at least 300-600DPI and A4 (210 × 297 mm) sized. Don't forget to proof and adjust it in CMYK mode after you've finished as the CMYK printing process will result in dulling of the colour. This is a limitation of that particular printing process and it's sadly difficult to avoid.
4. Learn HTML and CSS - Most of the bigger jobs I've gotten have been via my portfolio website, Loppiart. Not only this but it's the first port of call for potential employers too. It's your way of reaching out and grabbing them by the throat in a way that won't detract from your artwork, but will compliment it. You can, alternatively, do what a lot of artists do and use a plain, pre-made template with somebody else's credits on it or you can spend hundreds on having one created for you. But you know what's more fun, impressive, personal and looks great on your resume? Making your own!
5. Don't neglect realism - While sometimes it may not seem like it, everything I draw is heavily inspired by the real world around me. The fact is it's hard to abstract the human form well (and other things) when you don't actually understand its constituent parts. The same applies with colour. If you understand the way light falls on the surface of an object, you can then interpret it in a believable way with non-realism work. If you draw in a figurative way it will always be anchored to realism - so don't pretend it isn't important!
6. Reference Material - It's important to remember that you should never copy directly from reference material. You should LEARN how to draw the object by drawing studies and sketches from your reference material. When you've understood the form of what you're drawing, then can you seamlessly integrate it into your final artwork. I surround myself with dozens of art books and I collect images that inspire me!
Some useful ref books - [link] [link] [link] [link] [link]
7. Plan Your Picture - I used to sit down and draw a finished picture without any planning whatsoever. I did it A LOT too. Just doodled and erased until it looked passable and then inked and coloured. While this works for more dynamic and spontaneous pieces, it does very little for composition and overall cohesiveness. You don't have to plan in a neat and structured way, I get random bits of paper and scribble out thumbnail doodles that only I could understand. This serves as a guide to composition and perspective. I then plan out the more complex parts in detail and choose colours so that I know what I'm dealing with. I don't always do a final mock-up sketch but it's helpful.
8. Use a Board - When you're working on a traditional paper-based piece, use a large, rigid piece of wood, plastic or other smooth material to tape your paper too. Put masking tape over the corners to secure it to the board and your picture will be protected from tearing and folding. It also allows you to easily set it by when you're not working on it.
9. Don't be Precious - Try not to get too attached to your work just because you've spent a long time working on it. If it seems wrong, restart or rework the area until you're happy with it. This will help you learn from your mistakes, develop patience and a critical eye as well as overcoming sensitivity to constructive critique. Denial only limits you!
10. Bangin' Choonz - Make an awesome playlist of songs that provoke different moods. Mine is packed with classical and incidental music because I have a bad habit of singing terribly without realising when a song has lyrics.