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Less ideal method but still acceptable in a pinch: Start an overnight culture from a frozen stock, store that overnight culture at 4C, and use that overnight culture for further subdilutions and work. Usually, I use cultures left in the 4C for starting new cultures for about 3-5 days at most. Use the same overnight stock for your work. This is less ideal than the method above because the cells age and die with time.
Very bad Bad method: Subdilute a stock to create a new working stock, and then use that new working stock to subdilute into another stock, and so on. This is a bad method and should be avoided, as it can lead to accumulations of mutations and unknown changes that will cause fluctuations.
Maintaining Plasmids
In addition, although wild-type E. coli (such as MG1655 or EMG2) may seem to yield higher efficiencies for transformation, you should try to avoid maintaining plasmids in wild-type strains. Instead, use suitable cloning cells, which can have useful properties such as being endA deficient (leading to higher quality plasmid preps), defective restriction/modification systems, etc.
Tables of suitable cloning cells can be found online, such as http://www.neb.com/nebecomm/tech_reference/competent_cells/strainProperties.asp and http://openwetware.org/wiki/E._coli_genotypes