Washers and screws are easiest but a bit more expensive.....    Staples are built to spread and twist as they go in.  One leg going one direction the other leg going the other direction.... The idea is to make them very very hard to pull out.  I use staples when I need staples.
 
Staples have one leg longer than the other. Get the long leg started with a tap and a second tap should bring the other leg into contact with the wood.  once both legs are touching you can drive them in.  Tapping them closed a little helps too.  But I dont usually have time to mess with all the staples much.
 
deb