I'm having trouble with a few friends (I see on a regular basis) to take me, and what I say seriously. They sometimes straight up just ignore me. Also, it's not like I'm telling them to do stuff they can't do or things that are unreasonable, they're simple requests.
This weekend, me and 3 friends went out, got some drinks (only one for me because I had to drive home afterwards) and had a lot of fun. At the end we went to McDonald's to grab some food and we also took some of these balloons for kids with us. On my way home, I dropped 2 of the friends off, because they both live maybe a few hundred meters away from the McDonald's and the point where I dropped them off was on my way driving home anyways. So no big deal for me.
However, after I told them to pick up their trash and the balloons and they told me they'd do so, but after I arrived home I noticed they left it on the back seats and the floor. The next day I confronted them with that via text, both said it wasn't them. (Obviously it was one of them though...)
How can I make them respect what I say in order to take what I said serious?