Harvest seeds when the capsules are ripe and ready to shatter on their own. Place seeds on a paper towel to make sure they are clean and dry, then store in an air tight container (prescription vials work great. So do film cans, or the little vials that the glucose testing strips come in) and label with all pertinent information.

Store cool and dry, but these seeds do not have to be chilled.

I just snip off the capsules into a paper bag and keep them in a dry place. then I chill them real well before sowing them in late spring. They do best when cold stratified and they like warm soil to germinate.

If I want portulacas in the same place each year, I just let the seeds drop where they are. Nature brings them back for me.