Way back when I was defining everything, this is what I said:
For those God has chosen, He will also preserve them to the end, so that they will not fall away (they will remain saints).Thus far, (and I'm jumping around a little) we have God's eternal election (U) acting in time with an irresistible call (I) producing a faith in a hostile person (T), all of which are purchased by the death of Christ (L).
Because of all of this, one of the additional benefits of being chosen by a loving God before the foundation of the world is that the determining factor in us finishing the race is not us somehow maintaining our faith by our free will, but it's God hanging onto us and forging our faith in accordance to His perfect plan so it endures to the end. (P)
Now, this doesn't mean that every single person that has ever "prayed the prayer" or "signed the card" or whatever else is usually associated with conversion will endure to the end. It doesn't mean that some whom God has chosen may not stumble into particularly grievous sin (like David) and thus look like a non-believer. It means that those God has chosen will end up in Heaven. They will endure to the end, bearing good fruit and having lived a life that glorifies the Lord, all by His sovereign grace. At the end of the day, when they say with Paul, "I've fought the good fight, finished the race, kept the faith (2 Timothy 4:7)" the very next thought will be, "all by God's grace."
The Preservation of the Saints is also known by a few other phrases -
Once Saved Always Saved - Useful, but may give the impression that "having prayed the prayer or signed the card, always saved." Many have been given false assurance through a phrase like this.
Perseverance of the Saints - This is better, as now we're told how people who are saved remain and end up. They remain saints - set apart for God, and they persevere to the end. There's hard work and long trials ahead, but they can make it. Unfortunately, it can suggest that the work in enduring to the end is primarily dependent upon man persevering.
Preservation of the Saints - Vincent Cheung argues that this is the best summary of the biblical teaching on the subject. I agree. It tells us how believers endure to the end (by being preserved, not determined by them, but by outside them), and it tells us that they are preserved as saints, so that it's not a cause for free immorality.
Ok, hopefully I can finish up this series soon.