1. Linens: The first and most obvious choice is to put your towels, sheets, and other linens to use. Don't forget about things like shower curtains as well. They're great at placing between kitchen goods or wrapping flat breakable items like picture frames.
2. Clothes: Clothing boxes are ridiculously expensive. Although it's convienant to roll up to your new place and unpack things straight into your closet, clothing can save you big bucks in the packing material department. If you don't want to wash or iron everything, try using bulky items like sweaters or scarves to help stuff between breakables.
3. Pillows: From bed pillows to throw pillows and even your pet bed, pillows can be great at the top of the box to keep the pressure off the contents inside.
4. Zip Top Bags: Plastic bags are great at holding air, especially if they've been sealed with a line of tape in addition to their closure method. They're perfect for glassware and things that need pressure without any points that could press too hard on something fragile. Think about how Amazon ships things, you're just making your own inflatable pouch. Likewise, if you happen to have a vacuum sealer, those work amazing for this purpose.
5. Pantry Goods: Even if you're not keen on the idea of keeping your food mixed in with other things, bagged items like beans, rice and lentils can fill small nooks and crannies in boxes where items might otherwise shift. Plus, there's a good chance they won't be in your emergency food box anyway. Save the peanut butter for that box! If you're nervous, just put them in an extra bag. Oh, and don't forget, popped popcorn and peanuts in the shell work out brilliantly (plus you can still eat the peanut later while you procrastinate unpacking).