Charge your batteries and clear your cards.
That's right, simple things that get forgotten. If you are going to an event to take photos or even whilst on holiday remember to charge your batteries the night before, empty your memory cards (you should back up to at least 2 places) and then format the cards once you have backed up. Also remember to format in Camera, not anywhere else.
Change your perspective
If you take a photo and it doesn't look right, not exactly what you were after, then change your perspective. Either move to change the angle, get below your subject or get above your subject. You could even take a photo of the same image from multiple perspectives and then choose the one that best suits your need.