If you're making yoghurt (with or without a maker) its definitely worth trying different brands of yoghurt as starter - with unpasteurised goats milk, the cheap yoghurt from the Spar makes a much better result than nice Rachel's organic (which goes all stringy). Its just a question of hitting a brand that has bacteria that goes with your milk.
Not moving the yogurt while its yog-ing is also important, I believe, to get a good smooth result (otherwise the bacteria tend to break apart while budding). And as Pinkone says, strained yogurt is also very good.
So long as you start with the right temperature milk, and don't move it, you should get yoghurt every time . . .