So I'm sitting down to a nice bowl of c# spaghetti, and need to add something or remove something... but I have challenges everywhere from functions passing arguments that doesn't make sense, someone who doesn't understand data structures abusing strings, redundant variables, some comments are red-hearings, internationalization is on a per-every-output-level, SQL doesn't use any kind of DBAL, database connections are left open everywhere...

Are there any tools or techniques I can use to at least keep track of the "functional integrity" of the code (meaning my "improvements" don't break it), or a resource online with common "bad patterns" that explains a good way to transition code? I'm basically looking for a guidebook on how to spin straw into gold.

Here's some samples from the same 500 line function:

protected void DoSave(bool cIsPostBack) {
  //ALWAYS  a cPostBack
  cIsPostBack = true;
  SetPostBack("1");
  string inCreate ="~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~";
  parseValues = new string []{"","","","","","","","","","","","","","","","","","","","","","","","","","","","","","","","","","","","","","","","","","","","","","","","","","","","",""};


  if (!cIsPostBack) { //.......
  //....
  //....
  if (!cIsPostBack) {

  } else {

  }
  //....  
  //....
  strHPhone = StringFormat(s1.Trim());
  s1 = parseValues[18].Replace(encStr," ");
  strWPhone = StringFormat(s1.Trim());
  s1 = parseValues[11].Replace(encStr," ");
  strWExt = StringFormat(s1.Trim());
  s1 = parseValues[21].Replace(encStr," ");
  strMPhone = StringFormat(s1.Trim());
  s1 = parseValues[19].Replace(encStr," ");
  //(hundreds of lines of this)
  //....
  //....
  SQL = "...... lots of SQL .... ";

  SqlCommand curCommand;

  curCommand = new SqlCommand();
  curCommand.Connection = conn1;

  curCommand.CommandText = SQL;
  try {
    curCommand.ExecuteNonQuery(); 
  } catch {}
  //....
}

I've never had to refactor something like this before, and I want to know if there's something like a guidebook or knowledgebase on how to do this sort of thing, finding common bad patterns and offering the best solutions to repair them. I don't want to just nuke it from orbit,