A few that I've learned or developed over the years:
1) Always challenge your assumptions. Nothing gets you in trouble quicker than assuming something based on old or bad information. It's important to recognize when things are different than you expected them to be, and how habit patterns can get you in trouble in these instances.
2) Small sky, small planes. There are very few times when the hairs on the back of my neck are not standing up, telling me to keep my eyes outside looking. Partly this has to do with the kind of flying I do, but even in admin phases of flight (departure and approach/terminal environment) I have had more than enough close calls. In fact, the closest I have come to a midair has been in the GCA box at a towered airfield. Probably 5/10 of my closest calls have been around towered fields overall.
3) Use the boring moments to set yourself up for the exciting ones. System management, and getting ready for the next phase of flight. I learned this out of necessity, as you really need your navaids/approach stuff/radalt/fuel checks/etc done before you punch into the clouds as a wingman where you dont have time to do much other than fly form until you break out on final. That said, it is a good habit for any flying that I do. It gets my mind thinking about the next thing, and things always get busier with freq changes, vectors, alt changes, etc and having everything set up makes things flow much better and just helps me sit back and listen.
4) Piggybacking on #3, LISTEN. Probably should be #1, or even higher than #1, but I just got to it so this is #4. Always listen. Get proficient enough at the flying stuff so that you can do it while listening, and don't ever ever ever get into the habit of turning radios down to concentrate on what you are doing. The radio is your biggest tool for SA, regardless of what you do, or what other tools and goodies you have on board........I have them all, and they are all useless if I don't obey this rule. You always need to be listening and building a mental picture of what is going on around you. It makes everything else easier......whether it be decision making in poor/deteriorating weather, knowing what freq to expect next, what routing to expect, what conditions to expect at an airfield, etc etc etc.
5) It isn't over until you are parked in the chocks. There are a lot of things that can go wrong on the runway after landing. You wouldn't be the first person to die after going sideways off the runway at 50 kts. I always have to catch myself on the landing roll-out, and consider the folks before me who have died at such docile seeming speeds.