#1 Don't make unnecessary transfers
There may be weeks wherein some of your most expensive players are unexpectedly rested. The biggest mistake you can do is to panic and use up your free transfers or even take -4 hits to ship these players out.
In fact, them being benched (or 'Pep-ed', as many call it nowadays) once might be a good reason to stick with them as the odds of them being benched again reduce.
Save your free transfers until the manager press conferences are done. With so many crammed fixtures, players are more likely to pick up knocks and miss odd games. The manager press conferences generally happen a day before the game-week begins, and will give you a comprehensive update on player injuries.