It seems only obvious, but if you're still using that year old can of coffee you got on a great sale from Costco...well, I'll bet you can TASTE the difference stale coffee makes. And it's not a pretty picture - and it's not a pretty taste either.
Buy coffee as fresh as you can from a reputable store - coffee shops are often a great place to buy your own coffee beans to use at home. And buy in fairly small amounts, so you'll use up your coffee while it's still fresh and yummy. No point in making a great cup of Joe if the beans are as old as your gym socks. :)