When I look for a bag, I look for something with a dedicated laptop pouch and a buckle of some sort to hold it in place in the pouch. I also like to sleeve my computer for added protection. You don't have to pay a fortune for this as a basic neoprene sleeve usually does the trick and all but the cheapest "laptop bag" from Walmart will offer additional protection.
Just as an example of something that is pretty affordable:
Lenovo Messenger Bag