1. Set the level that is WAY above your normal painting quality and try to achieve it. It is doable although might take sometime.
2. Take your time yet try to enter into as many categories as you can.
3. Do not select you models based on the army you play. You will not be able to do it anyway, unless you play all three systems.
4. I suggest going for newer models. Many of them are WAY better then the older models, exceptions, of cause exist : Phoenix lords being old and all that, remain the best models GW ever made, but this is an exception that more or less confirms the rule.
5. Presentation, presentation and presentation. This is especially important in Duel and Open. The model should tell the story, base should be appropriate for this story, color selection should be appropriate for the story and be in harmony with the base. Style should be consistent. In other word if you go for a garish cartoony spacemarine don't give him very realistic well weathered bolter.
6. Use of advanced techniques is a must. Let me put it this way, GD judging is very technical. They look at the number of techniques used and how well they are executed. Free hand is a technique. Well executed one is a large bonus for you, poorly executed one can push you down. So, you need to judge whether you can execute something well or not, depending on this either use it or don't. If you know that you cannot possibly make smooth blending on the face and after several attempts it is not improving then a) drop it, b) postpone it and come back to it later
7. Conversions - see above. Poorly executed but well painted conversion will not save your hide. Do them ONLY if you are sure you can do them well. Don't do them just for the sake of having converted something. I had a demon for extensively converted mini, and another one for a mini with no conversion at all.