How is ignoring clear Biblical instructions in Leviticus justified? Leviticus 11:7-8
  
  And the pig, though it has a split hood completely divided, does not
  chew the cud; it is unclean for you. You must not eat their meat or
  touch their carcasses; they are unclean for you.
  
  Leviticus 11:11-12
  
  And since you are to detest them, you must not eat their meat ad you must detest their carcasses. Anything living in the water that does not have fins and scales is detestable to you.
  
  Leviticus 11:19
  
  Do not wear clothing woven of two kinds of material.


I haven't come across many Christians that campaign against the eating of bacon, the eating of lobsters/squid or that refuse to wear clothes made of mixtures of cotton and wool.

If a believer doesn't follow every instruction, how do they decide which to follow?
Note: I must credit the excellent book "The Philosophy Gym" by Stephen Law, from which these examples are taken
The typical reformed answer on this is that the Old Covenant is fulfilled in the the Death and Resurrection of Jesus and that we are now living under the New Covenant.
Lets look at this specifically point by point.
Leviticus 11:7-8 and Leviticus 11:11-12 have been fulfilled by by Acts 11:6-9

6I looked into it and saw four-footed animals of the earth, wild beasts, reptiles and birds. 7Then I heard a voice telling me, ‘Get up, Peter. Kill and eat.’
8“I replied, ‘Surely not, Lord! Nothing impure or unclean has ever entered my mouth.’
9“The voice spoke from heaven a second time, ‘Do not call anything impure that God has made clean.’ 10This happened three times, and then it was all pulled up to heaven again.

as far as Leviticus 11:19 is concerned this is pretty much the same line of thinking as above, its less explicit, but its essentially the same thing.