A lot of good responses on here, and I've got one more..... A good F.O. is a Captain in training. Act like it. Be as professional you can in all situations - even if the guy/gal to your left isn't. Think like a Captain. (That doesn't mean do the things Captains are required to do) Know the dispatch release. Become familiar with the MELs and what they mean. Don't assume the dispatcher and captain have verified any MELs that may exist and their expiration times. Question routing when there is weather. Do things that AREN'T required of you - like listen carefully to the Captain's briefing to the FA, balance the fuel often, and don't be afraid to ask the person sitting next to you about something you don't understand - if they're worth anything they'll be happy to explain it to you. And if something doesn't look right - whether it's on the walk around, ATC, gate agents, etc, go ahead and speak up about it - your Captain will respect you for having the guts to query outside party decisions.