This is the easiest technique of all.
When you introduce something new to your home, whether it be a shirt, book or toothbrush, just think "1 in 1 out".
Whether it be putting it in the donation, return, recycling or rubbish bin, get that duplicate item out of your life.
Resist the temptation to "keep it for spare". If you've lived with out a spare for this long, you will probably survive just a little longer without it.