My key learning points:
1) Hammer one bearing in all the way, don't forget the spacer in the wheel and then hammer the second bearing in so that it ONLY JUST touches the spacer and NO MORE. If you don't you risk putting undue pressure in the inner race and, especially with the cheap bearings, the 'give' may result in the bearing not turning freely.
2) As said earlier, don't use the old bearing after the new one becomes flush with the housing. Most wheel bearings have a space for oil seal (i.e. the bearing goes in further than the lip of the housing) and once the old bearing goes into the housing too it won't come out easily (it will be stuck next to the new one, hence the nightmare)! If you use say, an old socket to hammer in the bearings, be careful of the rubber seals in the sides of the bearings - the seals have a metal backing plate and if you dent this plate it will end up touching the balls and could affect the life of the bearing.