How can I improve the presentation of the food I serve?
Try to get some verticality in your dish. Everything just laid on a plate is boring. Instead stack your meat on top of your potatoes, stand a large asparagus spear upright, put some onion straws on top of your dish. Anything that gets some differing heights to your dish will make it more visually appealing.

Use your colors. Avoid dishes that come out with nasty colors, or do something to correct them. My wife likes to select vegetables that have complimentary colors. It makes the dish look more interesting than if it is mono-tone. You can sprinkle fresh herbs over the top to add some more color and flavor. 

Add some flair. You see this all the time with desserts in restaurants but can do it with thick sauces on a main dish too. Put sauce on your dish in a nice pattern and then run a tine of a fork through it at a perpendicular angle to make sharp streaks through it. Dollop a sauce in a spiral pattern in ever smaller amounts so that you get smaller circles around the edge. 

Watch Grant Achatz at Alinea preparing a dessert straight on the table. If this won't inspire you, nothing will.  He uses all three things I mentioned to make this interesting. Not to mention that he's doing it straight on to a silpat table cloth. And one of America's greatest chefs makes dessert for you table side, there's more than a little "presentation" in that alone. We ate there for our anniversary, and he signed a copy of his book for us, even his signature uses the circles and lines so common in dessert prep.