I clean my fans twice a year on my laptops, sometimes once every two years, and I almost never find a significant amount of dust, so unless you live in a dusty area, I doubt its because of dust collection.
Make sure you rest your mac on a hard surface and not something soft like your bed, couch, lap, etc. This is to ensure that airflow is optimal and also the the heat is not retained.
If something as simple as emptying the trash is significant enough to heat up your laptop, that may be due to either a low amount of RAM or a fragmented HDD (AKA it takes long enough to heat up, thus something is making it slow). So either upgrade your RAM (the max for your model is 6 GB) and/or conceder buying a [http://www.alsoft.com/diskwarrior/index.html|defragging program] for your mac or backing it up to time machine and reinstalling OS X and importing all your files back into it. If you have never reinstalled Mac OS X, fragmentation may be the biggest factor in this.
Another two options are to get [http://www.macupdate.com/info.php/id/23137/fan-control|Fan Control] to ramp up your fans as the computer gets hotter (as suggested by palokiller) or get a cooling pad, as theanar suggested