I find the best time to shop for the holidays is September - November. I have noticed that most items at great prices during the holiday seasons especially on Black Friday are usually older models. For example, I think you will find a great deal for Apple's iPhone 3GS this holiday because it is now two models behind. Most navigations, TVs on sale are all older models. If you don't mind this, wait until the Holiday sales to get what you need. I have found great prices for older model items that are of comparable quality to the newer model. Do your research on electronics before you buy. An educated customer gets the best deals.
For clothes I find the best sale on or before Black Friday if you want it for Christmas. After Black Friday everything goes up in price. If you can wait until January or February then you are golden since I have found most stores have GREAT clearance at this time.
Another great secret I have practiced the last few years is online shopping on Black Friday. You can get a lot of great deals without the lines and crowds. Beware websites might be slow or crash so the earlier you start the better. I usually start looking Thankgiving evening and get up early (7am) to shop online. I have gotten great deals on Amazon on Black Friday comparable to doorbuster deals at Walmart and Best Buy.
If you are going to go to the store to shop on Black Friday, go prepare. With the crowds and limited items it might be hard to just walk around and shop. Instead you should go to sites like this and look at all the sale flyers for black friday and make a list of things you want.
If you are home and have a few minutes, check frequently Amazon Daily Deals