I echo all the other things like sharp scissors, unpicker, iron etc - they're like extensions to my fingers and hands, but my favourite things are small pieces of cardboard.
When turning up hems or cutting off amounts over a wide piece, instead of using a tape measure or marking with chalk, I cut a notch in the side of the card (shaped like half a V) to correspond with the amount I want to turn up or cut, then line up the end of the card with the edge, moving it along and pressing or cutting as I go. I find this quick and easy and costs me nothing!