Packing is one thing that I both love and hate. Also, I am on one hand very good and on the other terrible at it. When I pack I bring everything, and I mean everything. In a way I am a good packer, I never forget anything. From bobby pins to yoga mat I got it (yes, I use to carry yoga mat in my suitcase). Some of you may already guess what is the problem with this way of packing, it's the weight. 
My suitcases used to be so heavy that I always paid for overweight baggage and was unable to carry them by myself even over one flight of stairs. So after one vacation I decided that this had to change and started to think about stuff that made my luggage so heavy. 
Of course first thing on the list was all that clothes I packed and never wore. 
Second was shampoo, conditioner, body lotion, toner and other beauty products and I always brought unopened bottles because I was afraid I will run out if I bring half-used ones.
Somehow I found a way to menage both of these weight causing factors and to make my suitcase a lot lighter. 
When it comes to packing clothes you have to make outfits. I like to find weather forecast for the place (or paces) I am traveling to, print it out and write what I'll be wearing next to the forecast for each day. This way you know what you'll be wearing every day and can pack those items only. Don't forget to match shoes and accessorize to your outfits. If your trip is short pack only two pairs of shoes (plus one pair of dressier shoes). This works for short trips, ones that last 10 days or less, for longer trips this method has no purpose, you'll end up with too much stuff. 
When traveling for longer period of time I suggest packing things that go well together and can be worn in different occasions. I don't mean T-shirt and shorts you can wear when you work out and again as pajamas (I actually read this advice in beauty magazine), that is just crazy. I mean a skirt you can wear during the day with simple shirt and with elegant blouse when you go out. This system saves me a lot of place in my suitcase. Another quick tip: if you know you'll be shopping a lot on your trip, bring outfits for the first couple days and for the rest of your trip wear things you bought there!
Not bringing unopened products with me was tricky at first. Until I realised I don't need that much product anyway! I learned that when I bought travel size cosmetics bottles. You can buy empty ones and fill them up with your own cosmetics. I got mine at Sephora, but you can buy them everywhere. 
Also, you can buy little travel size versions of many products. Before I took them on a trip, I tested them at home to see how long will it take for me to use all the product up. That way I know for how many days will ''minny versions'' last. And I have to say I was surprised, I actually need much less product then I thought. Another tip is to ask for samples of your favourite products at a store and take them on a trip instead of full versions. 
I hope these tips were helpful, if you have some other packing tips feel free to leave them in the comments,  I am also still learning how to pack more efficiently.