We've all heard the nightmare story:  A new parent, not used to having a child in tow, accidently leaves their sleeping baby in the carseat.  The parent goes about their business in the store, wherever, and then remembers later, or just returns to find their child dead or permanently damaged from the heat in the car.   When my husband and I just had our baby, we almost did this once (we remembered before we were 10 feet from the car) but the event scared me to death.  I stayed awake at night, re-living the moment, wondering "what if". 

So then, I came up with a method that, at least, worked for me for not ever forgetting, until I became more used to taking a baby with me wherever I went.  Maybe it, or some variation of it, will help you if you have the same fear.  I put a large baby bib in my child's carseat.  When I put my child in the carseat, I took the bib out and wrapped it around my wrist.  When I took my baby out of the car, I put the bib in the carseat (or the space where the baby carrier goes, if you have a carrier).  Then, when I put my child back in, the bib went right around my wrist again.  I knew that if I started away from the car without my child, I would feel the bib hanging down on my hand and it would be a reminder that I needed to get my child.  This bib never had to remind me as I was walking from the car, because I found that just doing the action of putting the bib on my wrist and seeing it on my wrist as I was driving was enough to remind me and get me used to having my child with me.  I never had trouble with this parental nightmare again!