1. just before you want to wear the item?
2. after each wash?
3. save it up for ages and then have a big ironing session?
4. none of the above?
I send towards #3, but sometimes this means I do #1 if I really want to wear something. And with my girls' dresses, I am not very inclined to iron them at all if they are just wearing them in the house and backyard, so try to get them ironed before they go out.