how to remove junk and leftover files

Hello,

I have an early 2009 white macbook with OSX 10.6.8 (fully updated). My problem is that in past 2 yrs the system file size has grown and I cannot find a way to get rid of all the leftover or useless files form my system. Because I installed an SSD in it, keeping an eye on drive space is getting important and it is now getting me worried.

Can you suggest me a good way to slim down my system file size without having to reinstall everything from scratch.

Thanks,