It's easy to work so hard to get a good result that you overwork your drawing or painting. Colours get muddy or they lose depth, the tonal range becomes more narrow or you lose the freshness or vision that you originally had.
There are a few tricks around this, particularly for students:
1. For paintings, spend 70% of painting time with brushes the same size or bigger than your thumb. Don't get too fiddly with small brushes - this is the surest way to make your work muddy and lose the big picture.
2. Always work from big to small. We often spend way too much time on the details, but the details are not how we orient ourselves through the world. We spend more time in the real world trying to not walk into a table than noticing the woodgrain. To convince someone of it's realism, the big structure has to be correct first.
3. Give yourself a time limit to finish the work or finish specific parts.
4. Try this technique: study your subject for 5 minutes, then draw for 5. Feel free to change or erase as you go. This can give some really interesting results.
5. Limit your palette (that is, the number of hues (colours) you use). Choose either a cool or warm palette at the beginning and stick to it (more on this later!) This includes blacks and whites. Avoid premixed tertiary hues (colours) whenever you can - make them up from your other colours. There are three reasons for this: you'll learn about colour mixing; similar colour combinations bring a sense of cohesiveness to your work (link parts of your work) and generally, the fewer colours, the fresher the work.
6. Step back to see, step forward to make marks.
7. Mix your colours on your palette, not your artwork.
7. Have a plan to blend colours or apply shading before you approach your work. Choose the direction of your strokes ahead of time and make them as efficient as possible. Strokes should show what the surface is like (the texture), and the direction the surface moves in (around, away, to the viewer, drops off sharply etc), as well as the lightness or darkness (tone) or overall colour.
8. Have an exit point planned before you start your work. In other words, have it clear in your head when your work will be finished (writing it down will help). You may like to think of it as the aim of your work, e.g. 'to create a strong, fresh floral painting.' This will help you know when your work is finished so you don't overwork it. As my favourite art teacher once told me, an artist knows when to walk away!