To kill dust mites in your mattress try using a steam cleaner with hot water on your mattress to help clean the mattress as well as kill off the dust mites.
You might also have to put some dust mite powder on the bed before using the steam cleaner to help kill the dust mites.
You can also just let the dust mite powder sit on the mattress for around 20 minutes and then just use a vacuum to suck up the dust mite powder off your mattress.
I've heard that using baking soda works good for killing dust mites as well and you use it just like the dust mite powder.