How to conserve very diluted plasmid?

[Olivier Friard]

Hi,
I want to conserve diluted plasmid (i.e. 1000 copies/ul).
What con I add to my dilution mixture (for the moment 20mM Tris.Cl pH 8.5)? 
DNA, RNA ...?

Thanks in advance (please answer on my Email too)

Olivier