You can't deadhead or you will be cutting off your future seeds. You have to wait for the seedpod to form and swell where the flowers were. The idea is to time it just before they split open and disperse. Often, you can still find seeds after they open. Here is a nice link with pictures so you know what it looks like. It's better than me describing it. You want to wait until the pod looks like it is drying out. Practice makes perfect!
http://doorgarden.com/2009/06/...
Here is a little video
https://www.youtube.com/watch?...
Pansies do take a while to get to the sizes you see in the nurseries.
http://doorgarden.com/2008/08/...
