Sometimes it's good to use simple maths too.

Look at your players' condition and then the time between matches, and work out how much condition they will gain before you play them again...that way you can judge wether you can fit a training session in or not.

If you're struggling with match schedules close together, I have found that at the start of the season, if you want to invest a couple of tokens to reschedule match times to suit you and your team better, the game will eventually adjust and schedule matches at similar times.

For example, all of my league games occur at around 11am GMT, and cup/CL matches at around 8pm GMT.

But as the others have said, it is always good to have squad depth, so you can rotate and rest fatigued players.