Try supplementation with glycerol or use "Terrific Broth"

Double the Chloraphenicol concentration in the last 2 h before the harvesting...2xYT might help too

You don't mention if you are using a kit for plasmid purification or a DIY protocol eg as in Maniatis et al. A coupe of suggestions:

1) For either method just do several plasmid preps, then pool and ethanol precipitate the DNA and re-suspend in a small volume of MilliQ or TE to give a reasonable concentration.

2) If you are using the Maniatis DIY method keeping everything cold makes a HUGE difference. Keep everything on ice throughout, chill pipette tips in fridge overnight etc. At the first step where you centrifuge 1 to 1.5ml of culture you can also double up the pellet size i.e. spin down 1.5ml, throw out the culture broth, add another 1.5ml and spin down again. DONT DO THIS IF USING A KIT tho, as you will exceed the capacity of the membrane in most spin kits.

3) Kits are less amenable to tweaking but with something like the Promega spin kit for example you can try using the midi prep or maxi prep kits rather than miniprep kits.

Hope that helps best wishes

Kim

To clarify, I am using Qiagen minipreps kits, with a yield of ~5ug per 50ml turbid culture. I typically use 3-4 spin columns (treating the pellet as if it were 5 ten ml cultures) with sequential loading (2-3 loads/column max) followed by ethanol precipitation to concentrate.

This amount of DNA is sufficient for one experiment, so I'm not totally up the creek, it is however a fair amount of work each time.

I will test different media (currently using LB) and spiking with Cam after o/n growth and report any increases.

Try using spectinomycin as well. I've used it to amplify plasmid yield for my p15a plasmid as well; it's the best alternative for when you can't use chloramphenicol. Just grow it up as you usually do, and after sufficient cell growth, add spectinomycin. Cell division will stop/seriously slow down, while the plasmid continues to replicate.