From the FAQ:

Q: How do I improve a players work ethic?

A: You can't - it's up to the player. Some kids will respond to playing time, some kids won't. Some kids will respond to starting, some won't.

That said, from my experience starts always seem to help with WE.  Whether that's because they are starting - or just playing more minutes than they likely would from the bench, I don't know...

As an aside, I have been using target minutes alot with my M2M teams (when I don't think it will cost me the game) to give lesser players starts, but less minutes than they otherwise would obtain on fatigue settings, and it seems to help WE.  I don't like to use target minutes ever with press teams though.

I had long assumed that starts always increased WE, but stumbled upon the FAQ above recently.  I guess it's possible that some players won't increase WE with starts.