The data side always needs to be kept away from any oil based substances, fingerprints or anything that can leave a residue. Most importantly, keep DVDs away from constant sunlight, heat and anything that could scratch them. However, unlike the old VHS cassettes, magnetic signals and strong electrical fields can not affect discs. These do affect videotapes, credit cards or floppy discs.
DVDs will get worn/dirty after time and there are products to clean discs. There are plenty of cleaning kits you can find online. However, soap and water can do the trick or rubbing in alcohol. Remember, anything acidic or corrosive should not be used. You should use a lint-free cloth and very gently wipe the data side of the DVD. A light brushing motion starting from the hub/centre of the disc, rubbing outwards.
As with CDs, it’s not the data on the DVD that gets scratched from use or neglect but the plastic outer coating that protects the data. Therefore, the scratch dissects the path, which the laser must travel in order to be read. To repair these scratches on the plastic, you can fill the scratches or rub them down with optical material. As mentioned earlier, there are plenty of cleaning/repairing kits online that one can use for this. Or even your regular toothpaste will sort it out!