We make our own film posters too, so grab their signature for when they are famous, and print a copy for them to take home too.With crew, it’s a “I scratch your back” sort of deal. Again, if you can’t fork out the cash, make a note of what they sacrificed when they helped you; the hours they worked, the cost and distance of their travel, any kit they brought with them, and their value on set. When they need you, match their value.If you aren’t paying your cast and crew, you should make their time on your film as comfortable and as convenient as possible.
If they need to leave early, they leave early, don’t fight it.
If they can only start late, that's perfect, they start late and you get a few hours with them.
If they need a lift home and there is a car available, taxi them home.
Make their time helping you as simple as you can. Always work around them, not the other way around. If they have to go completely out of their way to help you, they might not bother next time you ask for assistance.
Best of all, feed people. Free food is great, and if you ask them what dietary habits they have beforehand, it's a lot easier to please them. Pizza is a staple student film cliche. But it's not always the best option, so find out what their favorite meal is, and grab the cook book.