The biggest enemy to wine is oxygen, which can harm the freshness and taste of even the most beautiful of wines. Some wines can benefit from a small amount of oxygen before consumption as a way to "breathe" and shed the sulphur-like odours, but too much air and the wine starts to oxidise.
According to The Australian, once opened, wine begins to smell slightly nutty and the flavours start to disappear. Whites can take on a brownish tinge and reds can lose their beautiful colours.
So what do you do with your leftover wine to save it going bad?
Your best bet is to minimise contact with the air. Try to find yourself a smaller container in which to store your wine (the trick is to have as little air space between the wine and the top of the bottle) and then place it in the door of the fridge. This is even goes for reds. When you want to drink your red, simply take it out of the fridge 30 minutes beforehand to let it warm up.
Another option could be to freeze wine ready for use in cooking. It's a trick Nigella Lawson swears by.