Wish OSX were as rosy as these other posters would have you believe; it can use a bit of tidying up every once in a while. Defragging is possible, but typically not necessary in OSX because it is run in automatic scripts that typically run while you are sleeping. The problem is, especially with a laptop, your computer may be off when those scripts should run. I run Onyx about once a month to keep things neat:
http://www.titanium.free.fr/pgs/english.html .
Don't, however, feel like you need to run everything that Onyx offers. Don't, for example, delete ds files. Basically, just run the scripts and permission repair (which can also be done from disk utilities in your utilities folder).