1) Alternates

If you need an alternate, you need to plan for the the additional fuel to your alternate, plus 45 minutes reserve. Remember to add time for your climb, as well as an approach at your alternate. If you don't, you could find yourself it a very uncomfortable situation...

2) Holding

Holding...you usually can't plan for it. Whether it's unexpected traffic delays, lowering weather conditions at your destination, or even troubleshooting an abnormal indication, holding can have a major impact on the rest of your flight. As soon as you enter holding, make a plan for how long you can hold and still safely make it to your destination. It's always better to start your diversion early, than to sit in holding a get low on gas.

3) Improper Leaning

Forgetting to lean the mixture, or not leaning it enough, could cause you to burn a lot more fuel than you had planed for. Make sure you understand the leaning procedure for the power setting you pick for your flight.

4) Diversion

Even if you don't need to file an alternate, you need to be ready for unexpected weather along your route. If the weather starts deteriorating, make a diversion plan right away. The sooner you have your "Plan B" in place, the easier it is to make your diversion decision if you can't make it to your destination.

5) Unexpected Headwinds

Winds aloft forecasts have gotten very good. But that doesn't mean they're always perfect. When you get to your cruise altitude, check your actual groundspeed vs. your planned groundspeed. If you're significantly slower than planned, pick an early fuel stop in case the winds continue to stay above forecast conditions.