1.Don't turn the hand wheel backwards. Never. It should only be turned towards you.
This is something that comes naturally to me because I learned to sew on a treadle machine. I'm surprised at how many people try to do this. This is the best way to mess up the timing on your machine.
2.Leave the tension on your machine alone! The only reasons you should be touching it are for speciality feet, special needles or some machines ask the tension dial to be set differently for the button hole. First rethread, make sure the bobbin is in correctly, and make sure the needle is the right size and type for the fabric you are sewing. Otherwise leave it on the factory setting.
3.If you want to slow the fabric down, step lighter on the presser foot. Don't slow the fabric down by holding it back. You will get an uneven stitch and just might break your motor too.
4.The sewing machine sews in a straight line. You are not making the sewing machine sew straight, you are guiding the fabric under the presser foot in an even fashion. If you veer off correct gently, don't try to do it too fast.
That is all I can think of right now, I've been reading Yahoo answers and I'm not happy with some of the sewing advice that is given. I really have to stop reading. They are always telling people to adjust tensions! I RARELY touch my tensions. I have to stop reading them. Someone is insisting that you have to thread your machine with the presser foot up?!? Huh? What nonsense! As if that makes any difference at all. In fact, I'd say I'm more likely to thread the machine with the presser foot down! My Industrial machines have a knee lift, so the presser foot is definately down when the machine is threaded. And with my home machines, I put the foot down when I'm threading to get it out of the way of the needle. The only time I can think of it making a difference is if there is an auto threader that requires it to be up.
OK, I think I feel better now. Sigh....