Periodically, Steam runs ginormous sales of practically every game they sell. These Steam Sales usually come around in the summer (ie, mid-July) and a couple times in the winter (ie, Black Friday and Christmas). I love these, personally, since I can gorge myself on video games and store them up like a squirrel hoards nuts for winter. Although good deals abound, following a few simple rules can keep you from major regret down the line. I'm putting these together just before the Steam Summer Sale 2013, these rules apply to pretty much every massive Steam sale, ever.
1. Watch the Clock
Nothing sucks worse than paying for a game during a Steam sale, only to see it drop in price the next day. The # 1 rule of the Steam Sale is:
Only buy "deal of the day/afternoon/hour" games, until the last day of the sale.
So, don't rush out to buy a game the first day of a sale if it's not one of the games that has a "special" discount for only a few hours or a single day. The Steam Sale rolls over every day at 10 AM Pacific time, and each time it does new items go even further on sale.
Recent Steam Sales have even offered shorter time period deals, usually a few games for a few hours at a time. That way you're encouraged to check in with the sale periodically throughout the day. These sales tend to repeat, and sometimes there is a "best of" these flash sales as well. It pays to check in every so often, but don't stress if you miss a "flash" sale - chances are it will be around again later on.
If it's the last day, and there aren't any more daily deals or flash sales, now's your chance to pick up games that are just on a "normal" sale. Of course, if you're already full to bursting with new games, perhaps it would be a good time to check and see if you really need it at a "normal" sale price. Another Steam Sale is just a few months away, after all. The longer you wait, the less you pay.
2. Shop Around
Usually during the Steam Sales, other companies will get in on the discounts as well, and sometimes they undersell Steam.
For instance, Amazon has been running massive sales to coincide with Steam Sales recently. Sometimes Amazon's offering a bundle or a "complete" edition for cheaper than Steam is offering the base game. It pays to check in with the other retailers before you pull the trigger.
To coincide with Steam Autumn Sale 2013, Amazon is running a "Black Friday Digital Games Sale".
Amazon even tends to offer Steam game codes when you buy from them - just check the page before you buy, and if you see "Steam DRM" there, you're actually buying a key to redeem on Steam. You're saving money and getting the exact same product!
**Comparison shopping doesn't have to be painful - Redditor Moter8 points out a very useful site called IsThereAnyDeal. You can import your Steam wishlist, set price and store alerts, and manage the whole shebang from a very slick web interface.
3. Buy Gift Copies!
So a game goes on massive sale, and you're not sure if you are interested in it or not. You buy it, and it just sits in your Steam library for a year, while you scroll down the list and go "eh, not today." Before long, you've got a massive backlog.
There's a simple solution here - buy games as gifts instead. The gift copies end up in your inventory, and you can trade them for other games, in-game items, DLC, etc, with other Steam users. Usually these gift copies end up being worth a bit more than you paid in terms of trade value, as once the sale is over there are still likely people who want to own the game but missed the last major sale.
If you decide you want to play the game after all, no problem! You can just redeem the gift to your own account.
I bought this as a gift and sent it to my inventory, but note the "Add to my game library..." button.
The only downside is that Steam won't warn you if you're buying a gift you already own, so keep that in mind. You'll want to review your library carefully if you're prone to forgetting what you've already got.
4. Use Your Wishlist!
If you don't want to just mash the F5 key all the way through a Steam Sale, here's a trick - set up your Wishlist.
Beyond just telling your friends what you want to play (and what they should buy you for your birthday/Christmas, wink-wink-nudge-nudge) Steam recently added the ability to sign up for Wishlist Notifications which will send you an email when a game you're interested in goes on sale.
This can get kind of spammy during the Steam Sales themselves, but it's still worth it if you're prone to forgetting what you want to buy, and/or you want push notifications. You can't sit there and refresh the sale constantly during work hours, but if you get an email, you might be able to check your phone and buy stuff on your break.
With Steam Wishlist Notifications, you're less likely to miss the deal you're really excited about during a Steam Sale. Not to mention - sometimes Valve sets up special promotions that are only available to people who have lots of games on their Wishlists.
On top of this, when the Steam Store is overloaded, trying to click through to check prices on a dozen games you're interested in can be a real pain. Having it all on your wishlist page means that you can see every price you're interested in, in one page refresh.
In the "Shop Around" section I pimped IsThereAnyDeal and here again I'll mention it. It can significantly increase the utility of your wishlist if you spend the 10 seconds it takes to hook up to your Steam account and submit your wishlist to them.
5. Know Thy Sale
There are a few "Steam Sale Mechanics" that I'd like to cover. These are based on previous experience, and they may change if Valve or a publisher decides to break them. They have been consistent in the past though, so I consider this advice relatively safe.
If a game is on flash/daily/community sale, all versions of that game and all DLC are also on a similar sale. For instance, if they advertise Civilization 5 as being on flash sale, all the DLC, expansions, the Gold edition, etc, are all on the flash sale. It's always been safe to buy DLC for a game if the base game is advertised as being on flash sale.
This rule is also interesting because it means that you should always check the store page for a game if it's on "special" sale and you're looking for DLC. They might not advertise Dragonborn as being on "special" sale, but if the Skyrim Legendary Edition is, chances are Dragonborn is too.
Some games offer "upgrade" packs from the base game to the Gold/Game of the Year/whatever edition. For instance, I know Civilization 5 does, it's called the Gold Edition Upgrade. When the base game is on sale, the Gold Edition Upgrade also goes on a similar sale, giving you quite a lot of one-off DLC for a low cost.
Daily/flash/community sales are typically always the same discount on a given game. That is to say, if you see something 50% off on a Steam daily sale, and you think "I'll hold out for the flash sale" (or vice versa) - the flash sale will be the same discount. If it's on daily/flash/community sale, that's typically the cheapest it will be. (Thus the advice in tip # 1)
Occasionally pricing errors occur. If you see a ridiculous deal as a flash sale or daily deal, my advice would be to buy fast. I've picked up brand new games at major discounts this way, owing to some mistake on Valve or the publisher's part. Don't wait on a deal that is limited time but seems too good to be true!