I shop online quite a bit. Although the cost of shipping can sometimes cancel the savings, there are still ways that make online shopping very frugal and I try to take advantage of every one.
First, use a shopping bot like Bizrate or Pricegrabber. These shopbots search the web for the lowest price in the stores that are in their database. You can usually find a good price through them, but beware that they only search within their own databases, so check a couple of them before deciding.
Secondly, search for coupons or promotional codes for the product you want. There are sites that list coupons and codes for almost every retail site online, but be sure to check the date and any other limitations before trying to use them. Discontinued codes and coupons are usually removed, but not always. You will usually use these codes at checkout and they're usually good for anything on the site, unless it's restricted to a particular item. Combine these coupons with sales items and you can get a very good deal.
Another way to get coupon codes (some of them specific to you) is to sign up for site newsletters. These will often contain special savings, coupons, codes and "insider" news that will help you shop wisely.
Yet one more way and one that I've found is fun as well as money saving is to work a few "rewards programs" like MyPoints and Swagbucks. You can get gift certificates to your favorite stores and you're getting them free if you don't count the few minutes it takes each day to rack up points.
Online shopping has definitely come of age, so it's up to us to make the most of it.