Wearing same color clothes are a big no no. Pants and shirt colors should never ever match. They should contrast. Don't wear a blue shirt, and then wear a blue jean. You will look like you are in uniform.
If you do, stop wearing those "Traditional" Levis jeans. You know, the ones with solid colors and taper towards the legs.
Notice that most retailers sell jeans which have faded colors, have that "slept in" feeling, and are the "Relaxed Fit" cut. That stuff is in. Don't go into the extremes though as A&F does, theirs look second hand! Make sure the jeans are boot cut, and either relaxed fit or relaxed boot fit.
Vintage looking clothes are in, and the prep look is out. No more khakis and polos. If you have to wear khakis, don't wear those traditional Hagar and Docker ones. Look at their newer styles. Khakis are OK once in a while.
Long sleeve shirts with nice patterns are cool, like these. So are shirts with stripes.
Stop tucking in your T-shirts and polos. They should be a little below your waist level. If they are longer, then they are too big for you.
No huge jackets which make you look like you are packing 20 kilos of coke. Jackets should fit nicely. Same with sweaters.