Olson finishes up his tour of Calvinism (by way of "TULIP") in chapter 3 by describing perseverance of the saints (the "P" in TULIP). He defines it like this:
[A] truly elect person cannot ever be finally or fully lost because God will keep him or her from falling. (pp. 53)
I agree with Olson that this is by far the least objectionable doctrine in TULIP because it reflects little (if any) on the character of God. I have often been puzzled by this doctrine, simply because it is completely self-fulfilling (at least from our perspective) and provides little comfort or utility in the life of a believer or non-believer. Many examples can be given of "solid" Christians giving up their faith, and in these cases I often hear statements like "they either were not a genuine believer, or they will come back around eventually"... In either case, the doctrine is useless and provides little comfort.
I'm sure some people disagree; Perhaps someone who is absolutely sure of their salvation might see this as assurance that this will never change. But in that case, this assurance is only as sure as their belief that they are actually saved. In other words, telling someone that they can never loose their salvation is only comforting in as much as that person is sure they are actually saved. I've never seen this doctrine as adding anything to ones assurance that they'll end up in heaven someday.
Anyway, that end's Olson's tour of what he calls "Mere Calvinism", by way of the TULIP acrostic.