MBP Slowing Down

Firstly.

How big is the hard drive and how much free space is there on it?

What is slowing down and how?

I never heard of Mac slowing down after some use :/

I dunno if this ever helps but I have heard people recommending this a lot here on these forums.

Go to disk utility and repair the permissions. Also some people believe that downloading onyx and cleaning up the caches can help speed things up.

Lol well the Mac Pros are fast. But still Macbook Pros aren't slow.

To be honest I don't know where os x stores all the cache data. Maybe someone else can answer that. I know onyx does clean them all up anyway.

Cache is stored in the following:
username>library>cache

Clearing these can resolve various different issues however, this is not a maintenance step and should be approached with a certain degree of caution. Cache, after all is there to speed up your computer but flushing it every now and again when you run into problems/slow-downs can occasionally help.
To clear them, navigate to the above, back-up the contents and then trash them. These will be rebuilt on startup. This can take time and may slow things down until they are loaded correctly.

Repairing permissions as bmcgrath said, can also sometimes help. Other than that, run your cron scripts. Fire up terminal and run the following:
sudo periodic daily weekly monthly

These each do a variety of different things generally cleaning logs, backup netinfo & rebuild locate/whatis DBs.

You can do all this by hand, but as previously mentioned a GUI is often a simpler way to perform these tasks. Something like leopard cache cleaner, onyx, cocktail to name a few will perform these operations in a user-friendly manner but be aware of other features these suites offer as many do more damage then they cure.