The first toothbrush you ever used is still alive in a landfill and will be there for 100's of years!!
Engaging the brain is massive step. It's all about creating new habits.
Put things back if they are over packaged. THINK!!
Buy refillable water bottles and reuse them. Some tube/bus stations in London now have water filling stations (free) - write to local shops/shopping centers and ask for these. use them.
Take reusable bags to supermarket - weigh your fruit and veg without bags and then put them directly into the bag. Stick the price stickers on the outside, or directly onto stalk of brocc or potato.
Reduce what you buy - my Mum has just moved and was talking about all the linen she had to orgaise. Mum I said, it's just you. All you need are the sheets on your bed and a change of sheets. All you need are one or 2 sets of towels. perhaps a set for guests. We don't need cupboards full of linen.
If you want to buy something write it down. If you still want/need it in 10/20/30 days, then buy it.
Start up community sharing facilities for gardening equipment, ice cream makers & whatevers. We don't all need hedge clippers - we might just need to use a set once a month.
Recycling is good. Not needing to recycle as you didn't buy it in the first place is better.
Stop using fucking coffee pods, straws, etc