Several things have an effect on the ears. They can be taped as mentioned, and you also have to keep them trimmed correctly for them to stand up. Trixie's ears will start to flop if the hair on them gets too long.
Aside from that, floppy ears may also be the result of genetics passed down from the parents. If erect ears are a must as you've mentioned, I would first suggest not breeding a dog with floppy ears. If it is genetic, there will be nothing you can do to change them. I would start by trying to find out if this is a genetic issue with your female, or the result of them not being properly trimmed/taped. IMO, genetic backgroud is a MUST before breeding anyway.
