Some tips:

1. Look for a shop with a good return policy. Eg. Costco, and some mattress shops. With those, you CAN take it home, sleep on it, and then return it if not suitable.

2. Most newer mattresses are thick, and one-sided. So they cannot be flipped over to even out wear. That's not necessarily bad, though.

3. USA "King" size mattresses are pretty much square in shape (length == width), and can thus be rotated (weekly) by quarter turns to even out the spring/foam wear. This is a very desirable thing.

Cheers