In the most simple format, nitrogen is for sale in small containers that look a bit like a can of air freshener or something, and are supplied with a long thin nozzle to direct the nitrogen to the surface area of the residual wine. This is the best method I know of, but be aware that it is not foolproof. Because the wine has already began its degradation, there is no way to totally arrest this process, but this will have a significant impact on the time the wine will remain approachable. Ok, the bottom line. Once a wine is opened, it is destined to degrade.
White wines degrade faster than reds.
Once a white has been refrigerated, it should remain cool, or it's degration speed is increased dramatically. Reduce the headspace if you can. Use nitrogen if you can. Don't expect a white wine to last much more than 2 or 3 days. Sweeter wines tend to last longer.