The clear winner for tree padding, in my book, is a bath towel. You can buy long and fluffy bath towels for $1-2 at a thrift store and cut them into 4″x50″ (approx) strips. These wrap nicely around a tree, provide lots of padding, pack up in your gear bag without taking much space, and also keep your slings from slipping (unlike cardboard).

I see a lot of people not using padding, and it makes me sad. I was talking to a couple of girls the other day, who had setup a line on a tree in GG without any padding, and objectively I could see what I was doing probably sounded to them like lecturing. Tree padding is a serious issue, though, and it’s important to respect the environment around us. If you’re going to attach a slackline to a tree, please use padding. Doing otherwise risks killing the tree (yes, seriously). I know trees seem big and tough, but they have delicate circulatory systems just under their bark.

To summarize:

1. We don’t want to kill the trees; where would we rig our lines?
2. We don’t want to get kicked out of parks (and ruin slacking for everyone!) by angering park service / police / municipalities / etc.
3. We don’t want to damage our anchor slings by abrading them on tree bark.  At the least they will wear out and need replacement way sooner.  At the worst they might snap explosively and hurt someone.

This post on the slackline.com blog covers most of what I have to say on the issue, so rather than re-iterate what they said, let me just expand upon your options for cheap/free padding…

(gray Items in italics are already on their list)

• Cardboard – essentially free, and can be cut to any length.  For instance: [link].
  
• Carpet – I haven’t used carpet as I worry about the synthetic-on-synthetic friction.  Plus it’s heavy.
  
• Pipe Insulation – Popular for highlines.  Cheap, lightweight.
  
• 2″ Webbing – Protects your slings well, but doesn’t really “pad” the tree very well.  Not the best choice.
• Newspaper – Free (Seattle Weekly, Stranger), available everywhere, use when done reading, one Seattle Weekly will be sufficient for 1 line using 3ft wide trees, or many lines using smaller trees
• Firehose – I have yet to use this, and I suspect would basically just be a slightly more adequate version of 2″ webbing, as far as ‘padding’ value
• Towels – These are what I use most of the time; generally I use a couple of old beach towels I have cut into strips.  If you want to travel really light, cut them into 5ft long 4″ wide strips (or 8″ wide strips that you fold over once).  See anchors [link] and [link] or just look at the image above!  Towel strips are nice for rigging alone as you can wrap the towel around the tree once or twice and then “tuck the end under” in the same way you wrap a towel around your waist/midsection.  This allows you to start wrapping your sling without having to waste a hand holding the towel in place.

My only remaining suggestion is to please consider the padding benefit of the material you use.  Cardboard and newspaper are better than nothing, but not as good as carpet, towels, or pipe insulation.  I have used triple wrapped cardboard before (see above example), but consider towels to be a more effective pad (per square inch — and generally space in my gearbag is at a premium).