What is the biblical basis for the doctrine that salvation can be lost? In contrast to the doctrine of Eternal Security, many denominations believe that a person can, in fact, lose his or her salvation.

So, what is the biblical basis or biblical argument for the doctrine of the Loss of Salvation?  Put another way, what is the biblical argument against Eternal Security?
The Biblical Basis comes from the following verses (whether interpreted literally, figuratively, or out of context - these are the main texts used):
Core Verses
 - Galations 5:1-5 "fall from grace" 
 - Romans 11:18-22 being "cut off"
  - Rev. 22:19 unforgivable sins 
 - Ephesians 5:3-5, 1 Cor 6"9, Galations 5:19, Revelation 21:6-8 unforgivable sins
"Supporting" text
 - Matthew 6:5; 19:21-35; 10:22-32
 - Luke 12:41-46
 - 1 Cor. 15:1-2
 - Colossians 1:22-23
 - Hebrews 3:6, 14
 - 2 Peter 2:20-22

The biblical basis is clear based on these texts. 

In Gal 5:1-5, Paul warns of those who have "fallen away from Grace." 
Again in Romans 11:18-22 he warns not to be "cut off" and many people who believe you can lose your salvation interpret this to mean that God will "prune" the saved (those who have fallen away). 

Revelation 21:6-8 has a list of people who will burn in the fire and sulfur and those who believe you can lose your salvation state that once you sin after salvation you are adding sin to the Cross and therefore taint it again. Therefore, these verses show that if you are a liar you cannot enter heaven which means you have lost your salvation and need to accept Christ again. 

Again in Revelation 22:19 it says that "God will take away his share in the tree of life" which leads the reader to believe that someone who once was saved has been stricken from the "book of Life."

And of course Hebrews 6:4-6 where the author refers to "falling away" after being enlightened. This, as Drew put it, is the main text along with Mark 3:22-30 and Matthew 12:31-32 which cover the "unforgivable sin." They claim that if someone commits the unforgivable sin after accepting salvation then they MUST lose their salvation because the sin is, well, unforgivable. 

The other verses I have listed make small references to either falling away, lists of people who cannot enter the kingdom of God, or other minute details that might mean we can, in fact, lose our salvation. I hope that answered your question for the biblical basis/argument for it.