Trousers
You're probably wearing jeans right now - they're the most popular trousers in the western world. They're practical, durable and classless, but they do have a number of caveats to be aware of when cycling. The first is crotch seams. Many styles of jean have a thick seam exactly where you don't want one, which can cause pressure and chafing in a rather delicate area. Either choose your jeans carefully, or look for a cutaway saddle that has a slot down the middle - they're designed to alleviate pressure on your soft tissue, but they also do a very good job of preventing chafing due to trouser seams.About the perfect casual cycling trouser is a light cotton chino with tapered legs. Fortunately they're bang on trend at the moment and easy to find in most fashion retailers.
Tops
If you're comfortable in it you'll probably be comfortable on the bike, with the following caveats. Many bicycles put you in a hunched-over position, so you'll want to make sure that whatever you're wearing is long enough at the back to keep you covered. You'll warm up quickly on the bike, so wear less than you would normally - if you feel slightly too cold when you leave your front door, you'll probably be perfectly comfortable after a couple of minutes riding. If you're even a moderately quick rider, it's worth giving some consideration to aerodynamics. Lycra is obviously unnecessary, but anything very baggy will flap about like a sail and slow you down. Hoods in particular can flap up and act like a parachute. Be careful of anything that might become snagged or trapped on your bicycle - scarves are a real hazard and if trapped in your front wheel can cause serious injury.