I would definitely start with tipping the ears. That seems to help even though it doesn;t seem like the hair on the ears weighs that much..
I've also seen people tape the ears or even this method..
http://www.yorkietalk.com/forums/880898-post7.html
I have two puppies currently and when I got them at 12 weeks, one had ears that were straight from the start. The other one had ears that were floppy constantly. All I ended up doing was to keep the ears tipped and by around 16/17 weeks they have been straight up (most of the time). Their are both around 20 weeks now.