I have been watching Aunty Biscuit casting on a sleeve for the current jumper that she is knitting.  She uses the thumb method which seems to involve wrapping yarn round her thumb then pushing the needle adjacent to it and slipping the yarn through.  It all looks very convoluted to me, but apparently is easy once you get the hang of it.  Other methods of cast on can include effectively doing an increase into a stitch or cable cast on which seems to involve poking the needle between the two end stitches and knitting it.  This apparently forms a firm edge whereas the thumb method gives more stretch.  Mums preferred method of casting on is the cable method.  This is because she invariably doesn’t allow enough yarn to do the thumb method so has to pull it out about 10 stitches from the end, which is annoying when there are something like 300 stitches to cast on.  As the downside of the cable method is the firm edge, she often uses a size bigger to cast on before reverting to the proper size.