Hi,  
Probably you can pump a known current and sense the voltage drop across the resistance. There can be some feedback loop which generates an error voltage by comparing the voltage drop with a fixed reference. This error voltage in turn could be used to minimize resistance variation.

1 Meg to 30 Meg...

Yikes!

I do not know what you are trying to do, but if that is a resistance on a PCB, it is going to be sensitive to dust and humidity. It won't pass the "damp finger test"  ;D for a robust board design.

If that is a resistance inside a chip, unless it is a specialized process, it will get swamped due to leakage to bulk effects.

What are you trying to do here?

thanks,
Jerry