1. Research! Know what the best products are, and their retail/ebay/craigslist pricepoints. That will help you stay away from wastes of money, and to recognize awesome deals. I spent a lot of time reading reviews of 5 star products on Amazon, and reading through Mom Must-Have's on Amazon's Listmania. I also read through blogs and Mom forums. When I found something I knew I wanted, I checked the price on ebay and craigslist, making mental notes.
2. Best places for deals, in order: Garage sales, thrift stores, resale stores, Amazon Warehouse, craigslist, Ebay. Yup, Ebay ranked last. Oftentimes, with shipping and the competition for well-rated items, you're not going to get much off retail. Amazon Warehouse was a new find for me- basically they are reselling like-new customer returns, sometimes at a deep discount. I got the Angelcare Movement and Sound Sensor Deluxe (normally $115) for $46, brand new in the box. I've also seen the My Brest Friend for as low as $21. Not bad for a new item shipped to your door for free (you HAVE signed up for Amazon Moms already, right?)
3. Be picky. Every week there will be dozens of new garage sales and new items in at the thrift stores, so don't buy something unless you love it AND it's in great condition AND it's an excellent price.
4. Be a "regular". I go to the thrift store every Monday and hit garage sales every Saturday. I've developed my own price points for various items, and each week I find a nearly-new treasure.
5. If you spot an awesome deal on an item you already have, buy it and flip it. I like Craigslist best because Ebay can be a hassle with the shipping, fees, and disillusioned buyers. Almost every week I find something I can flip for enough to offset the cost of almost my entire shopping trip.