The biggest factor in the cost of buying a DVD is age. New release DVDs command a considerable price premium over titles that are six or twelve months old. Consider whether you really need to buy a DVD as a new release. If you've already seen a film at the cinema will you be ready to watch it again immediately upon release? If you have a pile of unwatched DVDs could you watch one of them now and purchase the title much cheaper later?
If you're favorite movie or TV show is released next week you're probably not willing to wait no matter the potential savings. In that case there are still things you can do to minimize the expense. Odd as it may seem buying a DVD immediately upon its release can save you a considerable amount over buying say a month after release.
The reason for this is simple, the week of release the title will get a lot of attention. Bricks and mortar stores will feature it in there catalogs and online stores will push the title heavily on their sites. This creates a lot of competition from stores all wanting to appear great value to you the customer. They offer you a hot new product at a heavy discount and hope you will buy other less heavily discounted items at the same time.
If you've decided not to buy the title the week of release it becomes a waiting game. Eventually the title you want will cost half or maybe a third of its original price. It's just a matter of trading off how much you're willing to pay against how long you're willing to wait. Finding out when the DVD you want is on sale can be tricky. Periodically checking the price at your local DVD retailer or favored online store is effective but can be a little tedious. Instead consider using an online bargain hunting website to help you find which DVDs are on sale at a given time. This will help you ensure you're always buying your titles at their lowest price since release. Choosing right site or forum to use will depend on which country you live in but Google should help you find something appropriate.
One final method of finding cheap DVDs is to look outside your own country. The same DVD is often priced dramatically differently in different countries. This works best if you own a multi region DVD player, but even if you don't you can still buy from other countries in the same region. For example Americans can look to Canada for bargain DVDs and people from the UK can look pretty much everywhere in Europe. You'll need to consider the increased cost of postage but there are often good savings to be made.