Could you post a short but complete program which demonstrates the
problem?

See http://www.pobox.com/~skeet/csharp/complete.html for details of
what I mean by that.

Do you know that some of the string variabkles can be released? If so, make
sure that after you have used them assign the reference to null. This way
there will not be any reachable reference to the string object and hence the
GC can reclaim the data:

class Test
{
  string str;
  public void Initialize()
  {
     StringBuilder sb  = new StringBuilder();
     /*create a huge string*/
     for (int idx = 0; idx < 100000; ++idx) sb.Append(idx.ToString());
     str = sb.ToString();
     /* oops a reference to a huge string is reachable through this object
*/
  }
  public void use_string()
  {
     /*use the string here*/
     /*finished with the string? then release the reference*/
     str = null; /*string is now unreachable, so it can be collected*/
  }
}

Note that this code is ugly and you should design your code to obtain
resources as late as possible and release them as soon as possible, so
really you should not have two methods like Initialize and use_string, but
instead have a combined method that creates, uses, and releases the string.

Richard