Ok, I'm having some Iss-ewes with my slow cooker.

For starters, my meat (cooked in fluid) keeps turning out hard, not soft and tender and falling apart. What am I doing wrong? Am I cooking it for too long? Not long enough?

Speaking of which, is there some guideline somewhere on how long to cook things? My SC only has high and low but I saw one on ebay the other day which had some sort of auto-turn-down to ensure your food doesn't overcook. Am I just using the wrong sort of slow cooker? Mine has a nasty plastic lid, not a glass one, does that make a difference?

Welcome all tips and advice you might like to offer...I'm ready and willing to be a SC devotee but my recent efforts have been disappointing.