Jesus' metaphorical use of salt and light to describe his disciples in Matthew 5:13-16 is one of the most familiar illustrations in scripture. It is also one of the most mishandled in interpretation, especially in interactive teaching settings. The usual procedure, often spelled out in Sunday school and youth group curricula, is for the participants to offer as many different properties and uses of salt and light as they can think of, then to find a "spiritual application" by way of analogy to each of these properties and uses. The leader is then to encourage the group to exhibit those applications in their own lives.
The problem with this type of interpretation, which amounts to allegorizing, is twofold. First, there is no control on the interpretations allowable. It is usually brought up that salt flavors food, acts as a preservative, was a valuable commodity, etc. But since there is no control--i.e., any property of salt is allowable--we may presumably find a "spiritual application" for the fact that salt is a stable crystalline compound, composed of the two highly reactive elements sodium and chlorine. Light illuminates darkness, makes vision possible, creates heat, etc.; we may again attempt to find a spiritual application for the fact that light acts both as a wave and a particle, and travels in a vacuum at a constant speed of about 186,000 miles per second. It may be objected that during Jesus' lifetime such scientific facts were unknown, but surely divine intelligence knew them when putting the metaphors in the Bible and knew that twenty centuries later we would learn them. At any rate, although such obscure facts usually do not come up in the context of an informal Bible study, the point is that there is nothing in the method to exclude them, precisely because the implied assumption is that everything about salt and light is somehow analogous to some aspect of the Christian life.
This brings us to the second problem with this type of interpretation: it ignores what the author (and in this case the speaker, Jesus) intended by the images used. It is not apparent, or even intrinsically likely, that Jesus meant that his disciples were similar to salt or light in every possible respect. Analogies normally resemble their objects in only one respect, or in a limited range of respects; usually the relationship is made clear by the context of the analogy itself. Therefore, the proper goal of interpretation in this case should be to discover in what way or ways Jesus' disciples are similar to salt and light. This is a point unfortunately lost even in some excellent commentaries.
The first thing that should be noticed is that the examples are parallel. After the analogy itself is made, the point made about both of them is that they can be made ineffectual, and lose any benefit that they may otherwise offer. Therefore, the most reasonable assumption we may make is that both of these analogies are being used to illustrate a single point. If then salt and light are analogous to us in a similar way, they would therefore be similarly analogous to each other. This immediately precludes most of the interpretations we may make based on the properties or uses of light or salt individually--for these two substances are distinctly dissimilar to one another. Jesus is making a single point in these verses, reinforced by using two analogies, but a single point is being made.
That single point is evident by what is done with the analogies: in both cases, the useful property of the element involved is lost or made ineffectual. In verse 13, the salt "loses its saltiness"; in verse 14, the possibility of the lamp being lit and "put under a bowl" is envisioned (in the form of a rejection). Therefore, whatever it is that salt and light do, there are situations in which they are prevented from doing it. That is the point: believers can become ineffectual in this world, and Jesus is warning them against it.
In the case of the salt, losing its saltiness makes it worthless (v. 13). Nothing can be done with it; it can only be disposed of. "How can it be made salty again?" is a rhetorical question; there is no answer. The point is that unsalty salt is useless to anyone--therefore precautions were taken to protect salt from humidity, which would leech it out. Putting a lamp under a bowl (v. 15) similarly makes it useless. No one can benefit from the light of a covered lamp. But everyone benefits from a lamp "on its stand" that "gives light to everyone in the house." It is for this reason--the common need for light--that "a city on a hill cannot be hidden." Jesus goes on in verse 16 to apply the analogy directly to his disciples: "let your light shine before men, that they may see your good deeds and praise your Father in heaven."
Herein lies the key to both analogies: letting one's light shine before people somehow involves letting them see one's good works--not in the ostentatious manner of the Pharisees (cf. Matt. 6:1-18), but in a way that gives glory to the Father in heaven. That is, what we are as believers--the fact that we are believers, and what that means in terms of what God is doing in our lives--must come out, find expression in our daily lives. This shouldn't have to be forced, but should be natural--as natural as a lamp being placed where it gives light, or salt being salty. Yet there is a danger that this will not be the case--else Jesus' warning loses its import.
If there is a difference in Jesus' handling of the two analogies, it would be that while the salt is actually lost, the lamp is only covered (although a covered flame, starved for oxygen, would presumably go out soon enough). Jesus may be saying that a disciple's testimony can be made ineffectual either by assimilation to the world (the salt losing its saltiness) or by hiding it from the world (the lamp covered by a bowl). But if such a distinction is made, it should be made on the basis of what Jesus actually does with the analogies, not on the basis of the intrinsic properties of salt and light themselves.
Examples of such overapplication of analogies are legion, and constitute a common fallacy in biblical interpretation. They have in some circumstances contributed to unbiblical Christian practice. One of the most common examples is that of "shepherd," applied (first metaphorically to Jesus) to Christian ministers. Everything that a shepherd does for his sheep is applied to the work of the pastor: feeding them, protecting them from enemies, leading them to water, etc. "Spiritual applications" can again be found for all of these aspects of shepherding. But careful study of how "shepherd" is used metaphorically in scripture indicates that it is used, in the Old Testament, never of the prophet and priest, but of the king; i.e., it is a leadership quality, not a prophetic and priestly one. In the New Testament, it is used primarily of leadership through teaching. Overapplying the shepherding analogy may be convenient for those parishioners who aren't interested in being anything but sheep, or for leaders who prefer to control every aspect of their people's lives, but it isn't biblical. It contributes to the overwork of pastors, the lack of responsible body ministry, and the general negligence of average Christians to grow into responsible positions of service.
In both academic and pastoral worlds, there is substantial pressure to come up with interpretations that are novel, creative, different. We want to pull out of a passage everything that may be gleaned from it. Unfortunately, we may at times overinterpret, discover meanings that were never intended by the writer, or the Holy Spirit. We need the discipline to distinguish between interpretations that are supported by the text and those that aren't.
 E.g., Carson, D.A., Matthew, in The Expositor's Bible Commentary, Vol. 8, ed. by Frank E. Gaebelein (Grand Rapids, MI: Zondervan, 1984), p. 138-39.
 Salt could lose its saltiness if a cake of salt containing impurities had the actual salt leeched out by humidity (Carson, 138). This is relevant to the extent that it makes the image itself understandable to modern people, for whom salt losing its saltiness is not a common occurrence. It would not do, however, to spiritualize the impurities, the leeching process, etc.