You need sniffer springs and a stronger shock. Springs are rated to weight/per measurement of compression. Say kg/cm or lbs/inch. This is called the spring rate.

The higher the number, the stiffer it is and more weight it can carry.

Progressive springs also work well, as more coils gets compressed, the effective spring rates increase. (trucks use leaf springs, which are a type of progressive spring). The problem with progressive coil springs is that they are expensive. Alternatively you can look for a set of adjustable height coil overs, with stiffer springs. Do understand, once you put on stiffer springs, the cars ride will be less comfortable if not loaded. For this some coil overs have a helper spring setup so if you car is lightly loaded, your ride won't go to shit.

Sorry I have no idea what the ratings on the springs on a myvi nor what is available in the market so you need to do your own product research.

This is just basic general info.