1. Construction

Always do this lightly on the page, it is essential for getting things right. It's the best habit you can develop and saves you a lot of extra work and time wasting. You can use a light pencil line and rub the lines out when the drawing is done or use a blue pencil which does not show up on scans and copies. This way you can sketch in lots of light trial lines until you find the right one without messing up your paper. Many artists use 'col-erase' blue pencils. .

Good examples of construction can be found in animated characters. Each figure has it own particular construction set up which is always used a a base and enables the animator to draw the character in any pose with the right proportions. Look at any quality animation character in several poses ,try to find some model sheets - there are many of these on the web- to see how characters are built. This principle can be applied to anything and is really essential if you want to draw things like buildings or cars. Everything can be constructed even things that don't seem to have any solid substance like clouds and rays of light.,
An essential part of constructing an image is perspective, if you want to draw realistic scenes then learn to get this right or your images will never be believable.

2. Simplify and Reduce

To draw from life you have to be able to simplify with your inner eye. Sometimes when learning to draw you come across things that seem too overwhelming to draw but everything can be broken down into basic simple forms, even the most complicated-looking things.
You have to take a bit of time to analyse things and learn to see through the clutter! Ignoring details when you start a sketch can be difficult so learn to reduce and draw the essential. If you learn to do this and practise it as much as you can then you will be able to draw anything in the end.
A good exercise is doing 30 second sketches because it doesn't give you time to bother about detail. Sketch as much as possible and even when you don't have a sketchbook handy, look at things around you and think 'how would I draw that?' if you do this alot, you'll end up seeing the world in a whole new way.

3. Volume

Try to imagine the volume and weight of what you draw, don't just draw outlines. When you do use a line to draw, volume can be shown just by varying line weight.
Why draw in 2 dimensions if you can make it look like 3? Place your objects correctly in their space and pay attention to the direction of the light source and see how shadows fall! If you are drawing from life walk around the model or object and see it from all angles. Look at the places between objects known as negative space, this can be very helpful in understanding volume and for measuring distances between things.
Using dramatic lighting is a trick you can use to make your picture 'pop', it brings a out strong contrast and drop shadows which often give the illusion of reality.
Another good trick is to use textures and patterns which can give an illusion of volume. Or of course you can go the whole way and render everything.

some helpful books:

Perspective for comic book artists by David Chelsea
Andrew Loomis anatomy books
Bridgman's life drawing

some websites to visit:

http://www.drawingboard.org/

http://www.thegnomonworkshop.com/

http://stevenegordon.com/

http://www.thegnomonworkshop.com/store/category/162/Drawing

http://www.myamericanartist.com/

http://gurneyjourney.blogspot.com/

http://features.cgsociety.org/articles/index.php?cat=tutorial

http://fossforslab.blogspot.com