After a walk around go and stand away from the a/c at a bit more of a distance, things can look different and is a good way of maybe spotting something that is not so clear close up!!
Aircraft laterally uneven- impending flat tyre or one tyre flatter than the other, fuel imbalance??
Antenna missing??
Bung left in or remove before flight tag??
These may seem like things that are obvious and could be spotted close up, well they can be, but if you look at the bigger picture, then on that 1% chance that any of the above are evident you have made sure of it...and also gives you a good chance of clearing your mind, composing yourself and being ready for the next situation instead of being in a dither
And on the subject of making sure, If you are ever unsure if you have or haven't done something, always double check it..
i.e. Sitting at holding point, unsure whether you put the fuel cap back on...instead of hoping to god you did, taxi back, shut down, get out and check, can be very annoying or embarrasing with pax, better that than having to deal with a serious fuel situatuon after take off...