Where do you put your helper classes?

Assume I have quite a few generic C# helper classes shared among indicators and strategies. Where do you put them to avoid duplicate copies (in indicator and strategy folders) with maintenance nightmare?

A related question is, how can one make use of all the nice features in C#??

For example I have a helper class called waveState that's something like :

 

Code

public class WaveST          {              public Pt P1, P2, P3, P4, P5;              public Boolean bull;              public Boolean meaty;              public List<Pt> Bumps;              public Cwave cw;              public Boolean inLTF;              public void Draw(int cr, Boolean e, Color c) ...

I have no choice but to put it in UserDefinedMethods.cs, but then I can't even use DrawLine call without getting something like "can't reference outer class methods from nested ... etc etc."

Other than no worry about memory management and the nice LINQ, it's like back to the days of straight C...