Darnit, people, just.say.NO! 
I wish I had. I helped a couple move twice and their apartments were awful. There was no animal waste, thankfully, but there were dirty dishes, old food in the fridge, used feminine products that hadn't quite made it to the trash can, clutter, clothes, and nothing packed. Clearly, I was an idiot
The first time I helped "Jack" and "Jill" move out of their 1 bedroom apartment, I didn't really know what I was in for. I knew they didn't clean much since the place was a roach infested firetrap, but they promised pizza and beer afterward. I was in college so that was the going rate for helping friends move. And I thought since surely they had moved before, they knew what needed to be done. Big mistake there. I knew Jack had started moving at 10 AM. So he said. Four other people were going to help them throughout the day and I arrived last. Jack didn't get a truck. His plan was to load up one large box, drive it two streets over to the new apartment, empty it, and bring it back, bucket-brigade style. When I got there at 6 PM, I honestly could not tell any work had been done, and not because the other four were being lazy. There was that much junk in that place and *nothing* I mean *nothing* was packed. Jill was nowhere to be found. At about 11 AM she had started to "feel sick" and was basically sitting in the other apartment reading while Jack and the other four (and now me) did all the work. We packed at least half a dozen 30 gallon garbage bags full of Jill's clothes (there were no boxes; I had to work with what was available). Someone thankfully went out and got more boxes for dishes and things like that. I washed their dishes and essentially packed their entire kitchen while everyone else wrangled with the clothes in the bedroom and the clutter in the living room. By the time we were finished, it was 11 PM. The stores were closed and they had not bought the beer beforehand. The pizza arrived about 11:30 and no one really felt like talking, especially not to Jill, who it seemed was feeling much better after the work was done. That pretty much ended Jack and Jill's friendship with two of the people who helped them move that day, and strained my friendship with them.
But as they say, "Fool me once, shame on you, fool me twice, shame on me." A couple of years later Jack and Jill were moving into a house. I made it clear to them that my help came with conditions - a) they would pack and b) they would rent a truck (the house was a 45-minute drive away). Jack assured me over and over that he realized the last time was a disaster and everything would be packed and ready to go this time. Myself and my BF were the only people from the prior move who agreed to help them the second time. Clearly everyone else was smarter than we were
Jack did rent a U-haul. But they were not packed. This time he had three large boxes which were full, but that didn't begin to hold all their junk. There were again dirty dishes, food that needed to be thrown out, used feminine products that didn't make it to the trash can, and, as Jack and Jill had gotten married in the interim, a bedroom littered with condom wrappers. I was *this* close to walking out again when Jack told us that he had to be moved that day because the lease was up the next day (a Sunday) and he had to turn the truck in by 6 PM. BF, who was closer to Jack than I was, asked me to stay. So again I stuffed clothes in sacks, threw away any paper that was on the floor (I may have thrown out mail they needed but I figured if they didn't think it wasn't important enough to sort, it wasn't important enough for me to worry about), and this time, etiquette gods have mercy on me, I packed their dishes dirty. There was no time to wash them and I did tell Jack they were dirty so he'd know to unpack them quickly. And where was Jill during this fiasco? She was at the house taking down wallpaper. She had "weak arms" (so she claimed) and couldn't lift heavy things and thought she'd do more good at the house. At the end of the day she'd taken down exactly three square feet in one room. I think the day was summed up when a friend, who was throwing out paper, found a dustbuster covered in dust hidden behind the couches. Jack didn't remember even owning one. We got everything moved and the truck returned on time. They halfhearted offered to order us pizza, but I just wanted to go home. That pretty much ended everyone's friendship with them, except for my BF, who is just that kind of guy.
So, in short, I let them take advantage of me (and so did my other friends). They were my friends, and there are things you do for friends. Of course, at the end of it, they were not friends any more. But they had shown they didn't mind taking advantage of their friends (given the state of the place for the second move), so maybe it wasn't such a loss.
But, yeah, I really should have said "no" at least the second time and probably "heck no!"
