First- Start with a fresh cut- and then immerse in water. This fresh cut should be done just before immersion- not more than an hour or two -or sap will prevent water absorption.
In the first 24 hours, a live cut tree can soak up over a gallon of water- so be sure to keep it hydrated.
Too much heat can dry out your tree, so avoid baseboards or fireplaces. The warmer your home is, the more water the tree will soak up, so consider keeping the room cooler.
Lastly, keep the trunk immersed in at least 2-3 inches of water at all times!