Make a Safe Approach and Entry
Don't wait until you're inside a structure to start looking for danger. Officers have been murdered by lookouts, getaway drivers, and burglars' backups that were positioned outside buildings. As you approach the scene, look for anything suspicious, such as vehicles parked where they shouldn't be. Use good cover tactics and stay alert for sudden threats.
Once you and your search partner are ready to make entry, do so quickly. Never linger in a doorway or other opening. If possible, utilize an entry point other than the one apparently used by the offender. He may be waiting just inside.
Announcing your presence as "the police" is a decision to be made on a case-by-case basis. If you feel there is a good chance innocents are inside, such as residents or a cleaning crew, announcing yourself and then waiting a minute or two or before entering is probably a good idea. If you're fairly certain only bad guys are inside and they're likely already aware of your presence, an announcement may only endanger you and your partners. (But in the case of a K-9 deployment, an announcement should always precede the release of the dog.)
If you are entering via an inward-opening door, throw it open forcefully to reveal anyone concealed behind it. You and your partner should move rapidly through the opening, wrapping yourselves around the doorframe to minimize exposure. (Decide in advance who will go first to prevent a collision in the doorway.) Once you are inside, keep your back against the wall and scan visually for immediate threats with your weapon in hand. Move quickly to solid cover and start your search from there.