As an AFOL (adult fan of Lego) who still builds every chance I get, here are my tips:
- Watch the sales. Target, Toys R Us, Walmart, etc. often have sales, or BOGO 40% off or some such. Esp. around Christmas and Easter. You can also find some sets on Target's clearance.
- Join the VIP program at Lego.com: If you have to pay full retail, at least this program gives you points back which you can then use for money off future purchases. (When they have double points sales is a great time to stock up. You can usually get two or more orders in during the sale period, meaning you can roll your earnings from order #1 into the next purchase and still earn more double points.)
- Garage sales/Craigslist, etc. Great for finding big lots of unsorted Lego. People just dump their kid's lego into a tub and sell if for whatever. You can sometimes score some gems this way because people sell sets and have no idea of their worth. (Side tip... get familiar with which sets/product lines command good money on the resale market and if you hit one of these gems, flip it for $$$ to buy more lego.)
- Bricklink.com. Great for finding specific pieces and used sets. Out of print sets can be pricey, but most individual pieces and used sets are pretty reasonable. Warning: Highly addictive.
- Ebay: Hit or miss, often times more trouble than it's worth, but there are deals to be had.
- If you have a lego store near you, they have events for kids that are either free or super inexpensive and the kid gets a small set to take home at the end of it. Check their calendar.
Once you have a big selection of parts, the buying should slow down because then you can build all kinds of things from what you already have. I say "should" because there are always new, cool sets coming out...
I spend way more than I should on the stuff, but Lego and board gaming are my only spendy hobbies. It could be worse, I guess.