I mostly make decks for fun in playing against my wife, not really competition grade stuff, more like between that and the premade decks. My current favorite is a Fairy/Steel deck to go against my Wife's Dark and Psychic/Poison deck. I try to synergize pokemon powers and effects with stadiums and tools. For instance, Mr. Mime or Shining Arceus can protect the bench against damage, or Aromatisse and Sylveon to move fairy energy around and heal, but also using Fairy Garden stadium to overcome the high retreat cost of Steel types like Mega Steelix EX. My basic structure is defense and healing, as well as taking out my opponent's bench. We also play in unlimited format, so sometimes we pick up older set cards to throw in, like Rocky Guard, I don't think that one is a legal play anymore with it being back from Plasma year ago, and I loved the two part Legend cards, Raiko and Suicune was my fav for my playstyle (really hoped Oblivion Wing would be like the game for Yveltal, but I don't think any renditions got the damage/heal at the same time attack).
I think most people tend to stick a few EX cards and a ton of healing/energy cards into a deck and try to sweep people out before they can build up anything, but that tactic isn't fun for me.