If you are worried about how cramped things are inside of your PC, an extra fan or two can really help move some air around. Getting the best out of these takes a little bit of thought, however; cases and components already have fans. You need to know how these work before you can decide where the extra ones will go.
A desktop almost universally takes air in through the front and exhausts it through the rear. Our Silverstone SUGO SG02-F also does this: 
there is one intake fan included with the case near the front which draws air in over the hard drives; and the power supply fan blows hot air rearwards. Considering all the stuff we’re jamming into this little case, that isn’t all that much air flow to go around. We will be adding an exhaust fan (using the bracket pictured below) in the upper rear, above the expansion cards. This will help draw warm air leaving the hard drive cage and air heated by the cards out of the PC.
In our picture to the right, we see the three important features to installing a fan, other than size: obviously if you are installing a fan in an 80mm bracket, it should be an 80mm fan. Use the screws that came with the fan. These are very different from the other screws we’ve been using; they actually cut threads in the plastic of the fan’s frame as they turn. You can find everything you need to know about choosing a computer case fan here.
All fans will have two arrows somewhere along the edge of the frame. These can be hard to see with the fan sitting in your hand; let alone in a picture, so I added yellow arrows over them in the picture. One arrow indicates the direction in which the fan spins. The other one is very important; it tells you which way the wind is blowing – literally. Use the second arrow to figure out which way the fan should face, depending on whether you want it to intake or exhaust.
Where you can (the small case we are working in limits our options), it is better to have more being sucked in than blown out. This doesn't cool quite as effectively, but, by raising the air pressure in the case, you make a little bit of air leak out thorough the cracks between your cases panels, drives and vents. That means dust will be concentrated around your fans, and be less determined to find its way in through every gap in your case. This is even more effective if your case or fans have dust filters.
Blowing out more than you suck in drops the case's air pressure, and this is a slightly more efficient cooling set up in a cleanroom. But in a normal environment, it sucks a tiny bit. Literally, it draws a bit of air through the cracks in the case, which makes for more dusty areas when your cleaning your PC.