Steam Sales Rules:
1) Only buy games on Daily Deals
2) Wait until the last day of the sale to buy a game you want that wasn't featured on a daily deal.
3) If a deal looks too good to be true, take it. It probably is a mistake. (see rule 4)
4) Secret rule number 4: 75% off is a good enough discount, it'll probably never get more discounted than that. The only scenario where it'll get cheaper is when the regular price drops outside of a sale and then it goes for 75% off again on the next sale.
Special Autumn Sale considerations:
Daily Deal prices might not be repeated on the Winter (Christmas) sale, so rule number 1 still applies. However, it has happened before.
Regular non-daily discounts will likely either be repeated or topped on a daily featured sale in the Winter (Christmas) sale. Since that is only weeks away, rule number 2, "wait until the last day of the sale", actually means "wait until the last day of the xmas sale".
Errors still happen, so if a newish game is listed at 75% off when the sale comes up or the daily deals refresh, you might want to buy it. It's probably a mistake.