I call ahead and ask if they have them in before I drive over. Sometimes they will say that they have them in a small trash bag and they just have to bag them up and to come on in, I just tell them to leave them in the trashbag and I will take it all and they are happy to not have to shovel it into those smaller bags.
A good time to get them is after the morning coffee rush, people on the way to work and all that, or weekend mornings.
I find weekdays are the easiest to get them since most people are asking for them on the weekends because alot of people do yardwork on their days off.