Hi, the most effective way, in my opinion, would be to perform  3x Ampure Bead cleanup and elute in a small volume of water.

Also in future elute using water if you expect low yields, even though product is not as stable.

Another option would be to precipitate your DNA again, cleaning and resuspending in low volume of water. Either manual or filter column methods. 

Simply precipitate your dna sample by Sodium Acetate and wash it by EtOH after that re suspend it in appropriate volume of water. 

Because you have a low concentration of DNA in solution, then Isopropanol precipitation would be a good alternative to ethanol as it precipitates DNA at lower concentrations compared to ethanol.

This question is answered nicely.

I too would use for the samples you now have the isopropanol precipitation.

And for future isolations, I would use the RNAse DNAse free MQ water that is normally provided with the kit and elute al lower volume. Make sure that the elution solution sits about 1 minute on the membrane of the column to give it time to dissolve the DNA from it. Alternatively you can reapply the eluted DNA again over the column, this should in theory increase your yield a little bit, but in practice I never got higher yields with that technique.

If money is not an issue, there are indeed kits that will help you clean up your sample and give you the oppertunity to elute in lower volume to increase concentration. But I would not do so since for every step you do, you will loose a little bit of your product (you never get 100% of it over at every step)

Thank you so much for the detailed replies. 

Add 1 volume of Isopropanol (Isopropanol is better than ethanol to precipitate little amounts of DNA) and 10% of NaAc 3M . Rock the solution.

Centrifuge at max speed for 10 minutes

Resuspend in 1 ml of ethanol 70%, centrifuge at max speed for 10 minutes. Remove the ethanol. Let the tube dry to remove traces of ethanol. (dont let the pellet dry, resuspend it moist)

resuspend in MilliQ