My gk has average form 7, but i found out on other teams that their form is sometimes weird...
I change most often mildfielders (mainly MC's) because they need all good skills, defs and attackers last longer in my teams.
Maybe this strategy with long-term players isn't perfect, but i found it as the best.
Spending 25+M on nordgen at levels 8 and 9 maybe seems like complete madness to someone, but it payed off through few seasons.
I spent then last coin to buy them, but it was good choice because they won 2CL's and Cup + played few semifinals...