Choose a team with lots of money, e.g Real Madrid, Barcalona, or Man United.

Don't buy any players at first. Instead go to staff upgrades and set fitness, goal keeping, defence, midfield and attack all to 10.

Then sell all the players you don't want or need to make the squad size 16.

After that play as many of your players as you can every season.

They will very quickly upgrade to 99.