If you can't afford travel insurance you can't afford to travel and if you are planning on anything out of the usual make sure your insurance covers it.
Photocopy all your documents and leave one copy at home.
Take sufficient tampons - it can be almost impossible to buy them in some places.
Observe local laws even the silly ones you don't agree with.
smarttraveller.gov.au is a useful website for up to date travel warnings etc
Make sure you have all your visas in order before traveling (some countries give you a visa on entry but you need to know which ones before you go).
The only customs officials in the world who enjoy a laugh work in NZ. If you're not in NZ don't joke with officialdom.
Do your best to learn local languages - just a few phrases can improve people's attitude towards you.

I wear sneakers and sandals on the plane - never had problems getting them on or off although they are a bit tight sometimes.