I've been a houseguest quite a bit this summer, and have spent a lot of time cooking in other people's kitchens.  Frankly, I'm surprised at how dull most people's kitchen knives are.  Not that I expect everyone to have a set of super-sharp Global Knives from Japan. The fact is, most of us home cooks don't need that kind of precision equipment.  But a quick lesson in using a sharpening steel (it's that round poker-like stick that came with your wood block set of knives) will keep your knives sharp.  Everytime you use a knife, the molecules at the edge of the blade come out of alignment, which dulls the blade.  A few quick strokes on a sharpening steel brings them back into sharp alignment.  For best results, you should do it every time you use your knives – but even doing it once a week (or month – or even for some of you, once a year) will dramatically improve your cutting performance. 

Serrated knives can't be sharpened this way althouth they don't dull as quickly as plain-edged knives. But when they do get dull, I'd recommend a professional sharpening.  (It's also why I recommend ceramic serrated knives, which hold their sharpness the longest).