I tend to keep things fairly simple, while accounting for as many potential problems as I can. It helps to spend some time thinking about what things counter your deck, and then include cards to address them.
Early on, having plenty of creatures is ideal, but once you have a solid card collection, a ratio of 20 creatures - 10 items - 20 spells is a good base. Some decks should have more creatures (like a Support deck), and you'll want to take the map into consideration... decks with more creatures and items fare better on small maps, but not as well on larger ones.
Element-wise, it really all depends on what you're aiming for. Overall, single or dual-element decks tend to be the most popular (and easy to play), but I've seen virtually every possible combination work well - even rainbow decks where the creatures were included based on their abilities and not their elements.
I like to think that how you play a deck is at least as important as how it's built. I've been in matches where a player's deck was countered very well by someone else's, but they won anyway due to playing better than their opponent. What I try to do is reduce risk... if you make something a juicy target, you'll probably lose it. If your deck is very aggressive and you play too conservatively, you'll probably have a hard time winning with it. If something on the map is a major threat and you do nothing about it... it'll probably cost you the win at some point. You get the idea.