I am new to the forum as I just bought my first Nissan this week, a 2017 Sentra SV. I bought it as an almost new certified pre-owned car - a dealer loaner car with just 900 miles on it.
Here is my problem. I decided on the Sentra because I had rented one for a few months and liked it a lot. In particular, I found the seats super comfortable on long road trips, something that is important to me.
While I was shopping for my car I drove several used Sentras with 5000+ miles and they had the same comfortable seats. However, I also tried driving a new Sentra, and found that the seat cushioning felt significantly firmer/stiffer, which was less comfortable to me.
When I first drove the car I ultimately bought, the one with 900 miles on it, I noticed that the seat cushioning also felt firmer, like the new car. However, I since all the cars I drove were identical (they were all 2017 SVs), and the only difference was mileage, I assumed that the foam in the seats would relax and soften with time.
However, I am a bit worried as in the meantime I don't find the seats to be as comfortable as I would wish - and as mentioned this was one of the key factors in my choosing this car over the competition. I am also a bit worried because I am not very heavy, and so I feared this might lengthen the break in period.
So I wanted to check in with other Sentra owners and see if you have had similiar experiences. Did the cushioning in your seats feel firm/stiff at first but then soften over time? And if so, does anyone know any techniques to hasten this process? For example, I have heard recommendations to soften new foam sofa cushions by walking on them. Of course this would be tough to do in a car unless one was very short!
Any thoughts would be much appreciated!
Sophie
This may be enough over time to "bust-it-up" , "break-it-in" .And an AC adapted 12V supply - you could run it for a couple of hours , better yet run that setup through a "Lamp timer" - shuts itself off !! 
That and throw some weights on top and its just like hiring a circus act ? 
G 3/30/2018