1. The 'guidelines' that govern 'was' and 'now only' prices in stores have become less clear in recent years. At one time a store had to have offered a product at the higher price for 28 days prior to showing a new lower price with a saving. These days many retailers interpret things differently and items that are half price may never have been sold at a higher price. So you need to check the 'current market price' and not be fooled by a huge saving that isn't real.
2. Owners of 'smart phones' like the Apple iphone have an advantage when it comes to checking the 'current market price'. There's a clever 'app' for that - when you're out and about in the sales and spot something that looks like a bargain, you can check the price there and then. Take a photo of the barcode and the 'app' will automatically look for search results on Google and comparison shopping sites and deliver the results to your phone instantly. You'll then know if the product in your hand is a true sale bargain or not.
3. Look out for 'loss leaders', most stores are prepared to lose money on one or two genuine reductions, just to pull in the crowds. Sign up to email newsletters and check voucher/coupon sites ahead of the sale start date to be sure you know what these are. You may also get advanced notice of sales and be able to beat the queues by shopping online. A lot of stores prefer to clear stock online than ship it around their actual shops.
4. If you miss out on the 'loss leaders' it might be worth waiting for the end of the sale when stores reduce prices further to clear stock. Don't be afraid to ask when a sale is due to end, most assistants will be able to tell you. This can also give you a chance to check online prices.
5. Don't be distracted by 'sale blindness' ask yourself would I still want this if it wasn't reduced? If you feel the urge - remind yourself that the reason the majority of products are in the sale is because no-one wanted to buy them at full price. It may be old stock or a piece of technology that simply doesn't work very well.