Think about the purpose behind the competition – if the brief is for an image that will be printed in a magazine, then make sure you shoot and supply images at a suitable resolution.
Is there a hidden agenda? For example, a travel company will want pictures that help sell and reflect their product as well as simply being beautiful.
As you choose your images, look for ones that have at least one special feature – is it a unique or unusual creature; does it show a funny event or a rare moment.
Trust your instincts – if you really like the picture, others probably will too.
Is it technically correct? Never send an underwater image that' isn't: out of focus will never win a competition nor will one that hasn't been lit or at least had a white balance adjustment. Slightly dark or slightly light can be corrected but be careful if you have to do more manipulating than that.
Be considered if you decide to manipulate an image. What looks good to you – and on your computer – may not look good on a different screen. Very few computer monitors are colour balanced and, although some machines will have a colour balance function, few people go to the effort. Laptops in particular can be deceptive. If you do decide to fiddle with an image, ensure you don't go so far that grain, noise or haloing appear. And be careful of saturating the colour to the point of it being rather unrealistic.
It's a good idea to crop an image if something weird has crept into your frame – like your buddy. But don't go too far and unbalance the image.