Stop applications that are unnecessary
First, see my relevant answer over here in regards to "killing tasks", especially the use of task killers.

The only way to stop these background tasks is to either:


Turn off any sort of background notifications the app does (check the settings for each app)
Uninstall the app or if you are rooted, you could "freeze" it with Titanium Backup.


Beyond that, you aren't going to be able to do much more. It's up to the programmer to decide when he needs to have a background task run, and a good programmer will let you have to turn these background tasks off (usually it is caused by polling for notifications).