You have to really torque down the fixing bolt. In fact to do it right, you need a torque wrench to know when enough is enough (sometimes enough is too much and you can twist the head of the bolt right off).

You should also grease the spindles to help the crank arms seat appropriately at the proper torque. Grease the fixing bolts too. If properly torqued, they won't come loose and you won't have to deal with threadlocked or corroded bolts later on when you go to change cranks.

Finally, if you rode them for any extended amount of time you probably want to pull them off and take a look to see if the hole has been pincushioned. This will happen if the crank is flopping around on the spindle under load.