Depending on your column I would refer to the GE SEC Chromatography Handbook (http://www.gelifesciences.com/gehcls_images/GELS/Related%20Content/Files/1466158841539/litdoc18102218_20161012165335.pdf). Chapters 5 and 6 refer to Sephadex. If you are using this medium for desalting or solvent-based separations the cleaning is different. I often reverse the flow at half the recommended flow-rate, flow water over the column for ~2 CV, then 1-2CV of 0.1M NaOH, 3 CV of water again, flow normal direction with buffer fo1 CV at half rec flow rate and 1 CV normal direction with buffer at the recommended flowrate. In some cases I increase the concentration of NaOH to 0.5M. Reversing the flow allows for junk at the top of the medium bed to quickly elute without traveling the entire length of the column. 

Again, I don't know what type of sep you are performing. The particle size matters as does the presence of solvents and what not. Make sure that you can reverse the flow of the column and not loose the medium. And do not flow at max flow rate. Best of luck.

As mentioned above you can use NaOH: in our lab we have been using it up to 1M. Remember to wash with plenty of water afterwards. In addition if you're going to clean an analytical column, you might want to change the filter on top of the column: it will be very beneficial for reducing back pressure. 

I separate condensed tannins in 750 ml wine samples using Sephadex LH_20 by using a gradient of Methanol (M) and Acetone (A).

I equilibrate the column with 50 % Methanol, then seperate

60M,

75M,

90M,

80M:10A,

65M:20A,

40M 30A

60A (Acetone elutes the condensed tannins I want)

How do I clean my column - I have been told that DMF or DMSO might work; what what I clean using the starting solvent? Would that do?

Assistance would be appreciated - thank you.