Question
What can I do to keep the flowers that bloom on my cacti to stay open longer?

Answer
Steven,

You can't. They are genetically (in most cases) programmed to only open for 1 or 2 days (max). The reason is that cacti are designed to conserve water, but flowers loose lots of water. So to minimize the loss, they open for a short time and usually at night (but not always depending on the cactus). Hope this helps.

Sincerely,

Greg