Buy the Daily Deals when you see them as they are normally the best price throughout the sale, they 'may' appear again in the final encore/repeat day but you wont know until that day so there is a chance you will miss out if it does not get repeated.
Don't hang around getting a game if it is a Daily Deal or Flash Sale as the discount does not always stay the same and it never gets better, only worse. I have seen games with high discount in a Daily Deal like -80% change within a matter of hours/minutes to a worse discount like -66%. They do change the discounts so be quick.
Dont buy any DLC's until the main game appears in a Daily Deal or Flash Sale as the DLC may get a discount the same as the main game. If the main game never shows up in Daily Deals or Flash Sales then buy the DLC on the last day. When the sale starts DLC on a lot of games will be discounted, dont be tempted to buy as I bet it will have better discount if the game goes on Daily Deal or Flash Sale.
Ive spent way too much time and money in the Steam Sales so I have personally been a victim of buying too early (worse discount, eg buying DLC on day one before the game was a Daily Deal) or buying too late (missing the better discount as I hung around too long.)
Basically, Daily Deal, Flash Sale or Community Choice - buy it as early as possible.
DLC or games that never have a Daily Deal, Flash Sale or Community Choice - buy it on the last day.
EDIT: Also, remember: Not all the 'deals' will be on the main page, pretty much everything gets a discount of some degree, search the store for what youre looking for. DLC is an example of this, in a Daily Deal it will only normally advertise the main game with a discount, look up the games DLC and see if that has been discounted too.