2. Throw the trash in dustbins (or carry a disposable bag) :
I carry a disposable bag along with me, which I use in case there are no dustbins around to throw the trash.
3. Do not accept plastic bags from shops :
People buy a small souvenir, and take a plastic bag along with it. Imagine the amount of waste generated only because of touristy shopping. I always carry a bag with me, so I don't accept plastic bags whenever I buy anything and keep the stuff directly in my bag.
4. Use public transport :
Using public transport may sound weird in case of responsible traveling, but it saves a large amount of carbon emmisons and also reduces traffic. Also, cutting down the use of flights can contribute hugely and can also save some bucks for spending elsewhere.