Aka free stuff: Paper napkins, utensils, cups, etc.
I usually buy my own cups but I usually get napkins and utensils for free. If I am getting food to go and there are utensils and napkins available, I go ahead and I try to take a little extra but without being greedy. If there is any leftover at the end of the meal, I toss it in my bag for future use.
If you aren't a jerk about it, this is a perfectly legitimate thing to do. If you are a jerk about it, it is borderline low level theivery and it gives homeless people a bad reputation. In other words, when you abuse such things, you make life harder for all homeless people, not just yourself. So please do not do that.
Courtesy items are all the more important when you are at your most desperate. So it is in your best interest to be judicious about it.