The easiest way to clear your ram is to get a task killer (there are tons of them on the android market). It works OK but can get annoying to kill the apps all the time, even if they automatically kill the apps. If you keep hearing from some people that you shouldn't use a task killer, they are full of crap. These are the people that are using slow phones and don't think they can get them to move faster, or at least have phones that have more ram and fewer preinstalled apps.
The best way is to uninstall them. You first need to root your phone. If you don't want to worry about installing a new kernel and such, the easiest way is to download Z4Root, which you can find here (it isn't on the android market): http://forum.xda-developers.com/showthread.php?t=833953
Otherwise, you can install a new kernel using this guide: http://www.howardforums.com/showthread.php/1676685-VM-Intercept-Rooting...
Once you have your phone rooted, download an app that allows you to access and edit the filesystem. For this I used Root Explorer. Go to /system/app and delete the apk of the app that you want to get rid of (be careful what you uninstall, some are required to have to be able to use the phone).
If you don't want to uninstall them, you can also get an app called autostarts (root required). Using this, you can keep them from starting up to begin with, and also keep other apps that you do want from opening and using up your memory so often (you still need to be careful with what you keep from autostarting).
Regardless of uninstalling the preinstalled apps or keeping them from ever coming up, you are still better off keeping a task killer at hand. Whenever you exit an app, it doesn't actually close the app. If you use multiple apps, they will all suck up the ram and slow down your phone. Using the methods above will just make it much more bearable.