1) Re-usable bags and more reusable bags! I have almost eliminated use of single-use plastic bags. I always carry my backpack around and keep a tote bag in it in case I pop into the convenience store for groceries. The same logic applies when I go to the supermarket for weekly shopping.
2) The same goes for plastic bottles (no more buying plastic water bottles). I always take my thermus everywhere now.
3) I don't have a car but I'm still mindful of taking taxis over public transport. Of course this doesn't mean I'll never take a taxi but if I can take the metro or a bus, I try to use that option more.
4) I haven't done this yet but I'm planning to experiment with solar panels. I live in a hot climate (we get the sun 300+ days here) so using solar panels to store energy to power small items like lamps might be a good way to save energy.
Would be interesting to see what other little hacks the community is using.