Plasmid DNA Purification - plasmid was contaminated by Genomic DNA (Jan/19/2005 )

hello, everybody.
i have tried three different kits for mini-prep plasmid DNA purification. but i could not get rid of the chrosomal DNA contamination though i strictly followed the manual of each producer.
could anyone give me any advice? thanks in advance.

Try to reduce the lysis time in step2. And do not extend 5min.