Here's a few wee things to keep in mind...
Have a look through some tackle catalogues (including Sportfish, of course). Look for the things they have in common, like action and close-ups. Not many shortbread tin sunsets in there, are there?
Don't make your shots too 'busy'. Three elements is a good target. Make it simple, and make sure it has a focal point.
Bear in mind the layout. If your subject is smack bang in the centre of a landscape-orientated shot, it is going to be right on the spine of the catalogue

And if you think that is stating the obvious, have a look through last year's entries.
Speaking of layout - think about where the Sportfish logo and other graphics are going to go.
Do something different.
Don't just look through your photos and send in the best ones. Go out there and shoot a winning entry. Leave the rod at home - it will increase your chances 100%.
When you have your entries ready, make use of the size limits for loading and display (758 pixels on the long axis and less than 100 KB). That means making your entries 758 pixels wide and 99.9 KB, and not 400 pixels and 30 KB
Hope that helps,
Col