To be honest, it's harder now that ds.com is all but gone (unless you live in the LA area). But there are still some things you can do...
1.) Keep your ear to the ground for new, hot releases and things people are excited about. Not all of them are hard to get or have to be fought for (mystery sets are popular right now).
2.) Popular characters! What you collect is not always your best trader. I collect Jess and Tink, but my best trader is Stitch. Popular pin themes tend to be Stitch, Jess, Tink, Tangled, UP, Princesses, Alice/Cheshire, sometimes Duffy and villains. That said, having a few of each theme is a good idea, although most of us avoid Fab 5.
3.) Ebay can help you with older LE 250/300 and even LE 100s for cheap. Not always, but sometimes you can find cheap lots (legit lots, not the mass scrapper ones). Just be patient and only snag the ones that are at cost or below.
4.) Just because something is LE 100/250/whatever doesn't automatically make it a good trader. Consider all aspects, like which characters and trade ratios.