A mistake I see pretty much every new player make is that they use Explore +1 +1 too much. It certainly has its place, especially when your handsize is small and you have no resources. In fact that's basically when you use it. If you have plenty of cards or you have resources, don't explore, or use Explore +5. Explore +5 is tricky. it seems bad but it's important for trying to see as many cards as possible to expand your combo potential. Explore +1 +1 isn't great. The main problem with Explore is that selecting it always allows everyone else to Explore, when it may be better to use other powers so that your opponent doesn't benefit so easily.
As a general rule, trading is awesome and I almost never win a game without doing it, unless I do a mass develop or mass consume x2 strategy. Trade for stuff. Don't feel like you have to go for alien (yellow) worlds, they're sort of a trap. Trading on brown and green worlds does just fine.