Wording for Declining an Invitation or Offer

• [Casual]  
  Sorry, I cannot make it.
  
  [Polite]    
  It is my honor to be invited to the dinner though, I would not be able to join it.

• [Casual]  
  I cannot take your offer.
  
  [Polite]    
  I appreciate your kindest offer though, I would not be able to accept it.

• [Casual]  
  I cannot accept your proposal.
  
  [Polite]    
  Your proposal looks very attractive though, I am afraid I have to decline it.