It's really annoying when a DVD freezes and skips on you, just as you are enjoying your movie. Usually such skips and freezes are caused by a dirty DVD.
Luckily cleaning DVDs is easy. You can get a clean DVD with just clean water and a lint free cloth. (Or even a paper towel.)
Inspect the DVD on both sides. Most of the time DVD freezes are caused by debris, but if the DVD is physically damaged, there may be nothing oyu can do. Damage on the top side (where the title and art is printed) can actually harm the DVD more than the bottom. This is because commercial DVDs are dual-layered, and the second layer of image may be closer the top and the bottom.
Luckily, most of the time, DVD freezes are caused by crud. If you discover fingerprints, pizza gunk, or smears of toffee, you can pause to wonder just exactly what the previous users had been doing with that DVD. Then take it to the kitchen so you can start cleaning DVDs. (As long as you are doing it, you might as well check any other questionable DVDs to see if they need cleaning too.)
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Get your cloth. You can buy a lint free cloth at a camera shop or online. Some video stores may carry them too. However, I often use a paper towel, because it's handy and it doesn't shed too much lint.
Do NOT use tissues or toilet paper, or even many kinds of paper napkins. These will leave a lot of lint behind and can make the DVD worse than before. (Although you can clean it again if you do end up leaving lint on the DVD.) If you use paper towel, be careful about rubbing too hard when cleaning DVDs -- paper towels are scratchy.
Run water over the dirty surface of the DVD. Tap water is probably okay, but if you want to be really careful, use distilled water. It should not be too hot, because that could cause the plastic to warp, and the laminate to come undone, but warm water will help dissolve sticky stuff better. It also helps with oily residues.
Wipe the crud off the DVD carefully, from the center out.
IMPORTANT: DO NOT WIPE ROUND AND ROUND! You could do more damage than good!
DVDs have special back up tracks built into them. That way if a tiny bit gets damaged, it can find the same information elsewhere on the disk and you have no problem. However, if that data is usually stored further along the same track. So if you wipe around the DVD, and there happens to be a little grit on the rag, you could possibly damage both the original and the back up of the data. If you wipe outward from the middle, and something gets scraped, odds are there will still be back up data.
Don't put too much pressure on the cloth. Do not scrub. Just let the cloth soak up the water and dirt as you wipe. You may not end up with a clean DVD on the first try, but you can run more water, and wipe again.
Follow these steps and you should have a clean DVD playing in no time.