Runway incursions, a big topic according to the FAA and it should be. I've posted tips & techniques on helping prevent incursions before but here are a couple of big ones:
Unsure of where you're at or how to get to where you want to go? Ask for a PROGRESSIVE TAXI. The controller will guide you step by step.
If you come upon Runway Hold Short markings and you don't have a clearance to cross or you're not sure if you have a clearance to cross, STOP!
I've been lost on an airport before, it's not fun. I've even cross a runway without a clearance (got a good ass chewing too). The common factor in both of those problems was I was distracted. I wasn't paying good enough attention to what I was doing and I screwed up.