If unconditional election is true do we worship God in vain? According to the teaching of Unconditional Election, God has already chosen who is saved and who is condemned. If we have no control over whether we receive or reject grace, someone can never truly know if they are saved or condemned. Therefore why does a Calvinist continue to worship and serve God if they don't know if they will be saved? Aren't they worshiping God in vain?
If we have no control over whether we receive or reject grace, someone
  can never truly know if they are saved or condemned.


Reformed theology teaches the doctrine of assurance of salvation, in addition to the doctrine of unconditional election. This doctrine teaches that we CAN know if we are truly saved or condemned.

The basics of this doctrine can be found in the Canons of Dort:

Canons of Dort, I.12 and V.9.10.11.13 


  The elect in due time, though in various degrees and in different measures, attain the assurance of this their eternal and unchangeable
  election, not by inquisitively prying into the secret and deep things
  of God, but by observing in themselves with a spiritual joy and holy
  pleasure the infallible fruits of election pointed out in the Word of
  God such as, a true faith in Christ, filial fear, a godly sorrow for
  sin, a hungering and thirsting after righteousness, etc.


It can also be found in the Heidelberg Catechism:


  Q21:  What is true faith?
  
  A21:  True faith is not only a sure knowledge, whereby I hold for
  truth all that God has revealed to us in His Word, but also a hearty
  trust, which the Holy Ghost works in me by the Gospel, that not only
  to others, but to me also, forgiveness of sins, everlasting
  righteousness, and salvation are freely given by God, merely of grace,
  only for the sake of Christ's merits.


It can also be found in the Westminster Confession of Faith, in Chapter XVIII: "Of Assurance of Grace and Salvation".

Essentially, by seeing in ourselves the signs of salvation, we can know that we are saved. Based on this knowledge, we can from there continue to follow the commandments of God in the assurance that our worship is not in vain.

It could be also argued that it is in every way preferable to continue to worship and obey God even when we doubt our own salvation.