How do the proponents of the idea that salvation once received cannot be lost explain Hebrews 6:4-6 and 10:26-29? How do the proponents of the idea that salvation once received cannot be lost explain the following verses: 

Hebrews 6:4-6: 


  For [it is] impossible for those who were once enlightened, and have
  tasted of the heavenly gift, and were made partakers of the Holy
  Ghost, And have tasted the good word of God, and the powers of the
  world to come, If they shall fall away, to renew them again unto
  repentance; seeing they crucify to themselves the Son of God afresh,
  and put [him] to an open shame.


and Hebrews 10:26-29:


  For if we sin wilfully after that we have received the knowledge of
  the truth, there remaineth no more sacrifice for sins, But a certain
  fearful looking for of judgment and fiery indignation, which shall
  devour the adversaries. He that despised Moses' law died without mercy
  under two or three witnesses: Of how much sorer punishment, suppose
  ye, shall he be thought worthy, who hath trodden under foot the Son of
  God, and hath counted the blood of the covenant, wherewith he was
  sanctified, an unholy thing, and hath done despite unto the Spirit of
  grace?


It seems to me that both places are about Christians who had once received salvation, but later lost it due to their change in mind or in faith.
When looking at the overall context of all the warnings in Hebrews we have a situation for the unpardonable sin of fully rejecting Christ even under the sanctifying work and demonstrable power of the Holy Spirit making it absolute, thus unforgivable.  It represents a full grown and stubborn unbelief in the face of a long season of powerful grace and the Spirit's manifested power.

All one has to do to understand these verses is to 'think Judas'.  Although never properly sanctified by inner regeneration, his mind had been in a sense 'sanctified'.  What confuses some is they automatically think a person can experience no kind of sanctification except after justification but this is not correct. For example a 'unbelieving' spouse is sanctified in some ways by the believing spouse to make a marriage holy even when one spouse is not inwardly sanctified. 


  For the unbelieving husband has been sanctified through his wife, and the unbelieving wife has been sanctified through her believing husband. Otherwise your children would be unclean, but as it is, they are holy. (NIV 1 Corinthians 7:14)


In the same way by being with the Lord every day, having himself been given gifts to preach and to cast out demons when the Apostles were sent out two by two, Judas had 'tasted of the heavenly gift'. Judas was made 'partaker of the Holy Ghost'. Judas certainly 'tasted the good word of God, and the powers of the world to come'. However, Judas was a bad field that did not produce a good crop and was never 'saved'. Therefore, as Judas 'fell away' it was really not possible to 'renew him again unto repentance' seeing that in his unforgivable rejection of eternal life, Judas was  'crucifying the Son of God afresh' leading him into the bowels of hell, which he himself under the horrible misery of sin cast his own life into.

The very same dangers faced many of the Hebrews soon after the resurrection of Christ.  The gospel had been recently been received by many Hebrews, but their faith was uncertain and some were tempted to reject Christ over Moses.  Such wickedness was equal with Judas' rejection. Even after they had witnessed the power of the gospel in an unusual manner.  Not only witnessing the outpouring of the Holy Spirit, the evidence of the risen Christ or testimony of Apostles concerning it.  They also had the word of God proclaimed to them in fulfillment of all that the prophets had spoken and now unveiled by gifted men animated by the Spirit and wisdom and knowledge.  If they were to reject Christ, in preference to Moses, they would be no different than Judas.

Even the history of the first generation of Jews under Moses typified their situation and Hebrews recalls the story forming the context of these verses.  For example, 'What is the deliverance from Egypt but a display of God power?' Did not God provide a means to deliver the minds and bodies of the Jews from the hands of Pharaoh by his mighty hand and then present an opportunity of faith during the testing in the desert? The first deliverance through the Red Sea did not represent regeneration, the second crossing through the Jordan did, as only then did they entered a land flowing with milk and honey, wherein the battle of sanctification would begin. (i.e. the kingdom of God) How did those Jews respond to this tasting of the power of God and sanctification caused by the release under the tyrannical powers of Pharaoh? (Pharaoh is representing the Devil's malice and power to blind a man from having opportunity of faith and rest by faith).  Well they did not believe, so God destroyed them! In other words, nearly the whole generation was a Judas, a reprobate, an unsaved wicked sinner despite the power of God in saving them externally and sanctifying their minds, in order to have the option of worshiping and serving the One true God by the rest of faith.

This is the context of Hebrews and all the warnings in it.  Early on in the book we find this argument so clearly laid out.  Therefore only those who have genuine faith proven by souls that do not fall away are saved. Those who never had faith prove it by their actions. No matter their confession the Jews in the desert were unwilling to believe so they died:


  See to it, brothers and sisters, that none of you has a sinful, unbelieving heart that turns away from the living God. But encourage one another daily, as long as it is called “Today,” so that none of you may be hardened by sin’s deceitfulness.
  We have come to share in Christ, if indeed we hold our original conviction firmly to the very end.
  As has just been said:
  “Today, if you hear his voice,
  do not harden your hearts
  as you did in the rebellion.”
  Who were they who heard and rebelled? Were they not all those Moses led out of Egypt? And with whom was he angry for forty years? Was it not with those who sinned, whose bodies perished in the wilderness?
  And to whom did God swear that they would never enter his rest if not to those who disobeyed?
  So we see that they were not able to enter, because of their unbelief. (NIV Hebrews 3:12-19)


The truth is those who believe that every true believer will persevere by faith (or once saved always saved) these verses pose no difficulty whatsoever.  On the contrary they are the most clear descriptions of cursed men who produce nothing but thorns even when tasting the Spirit in external convictions, powerful displays and convincing demonstration on the truthfulness of the gospel, which they fully reject.