Start thinking about what you would do in X situation as a captain early on when you are an fo is a good idea. Basically don't just sit there and be a bump on a log with the "its not my problem" mindset because you're not the one with 4 strips until you are in upgrade training. Being an active an engaged crewmember is beneficial for the whole crew.

Showing up prepared goes a long way, as others have said. But also having a plan and thinking ahead is also important. I say this because I have had many an FO on OE give a blank stare when I ask them "how are we getting down, whats your plan" when at 30,000 feet and I know the "Pilots Discretion to 5,000 feet" call is coming up later from ATC when doing a turn to an outstation. The "descend via" profiles out there into bigger airports today are generally easier, its the "when do you want to slow down, descend below 10,000 within an acceptable distace to the airport" that fudges people up.

Have pride in your look. Don't show up to work looking like a slob.

And while this job can be stressful and suck at times (commuting, getting that horrible line you really didnt want, flying with that CA/FO/FA that you really dont like, etc), reminding yourself that "damn, this is actually the best office view in the world and i get paid to do this?!" can keep things fresh for you. It is easy to get wrapped around the axle with some crappy parts of this job that people forget that we are doing some people's dream job.