My decks tend to be in one of two forms, the fun deck, or the anti-popular deck. I actually play fun decks in tourneys as much as anti-popular decks. The way I come up with these decks vary greatly.
The fun deck is usually a whim, a "you know what be cool..." kind of thing. I remember back around CC I thought it would be cool to build a deck focused around those laughed-and-mocked PB Vader's and Luke's. From there I said, how can I make such a deck work. I looked at cards and combos and decided the best way to do such a deck was to get them to battle me then make them pay for it. So I went about tossing in Vader's, Comlinks, Stormtroopers, Trooper Assaults, DJ Presences, and You Overestimate Their Chances. For LS I did a similar, albeit more complicated thing. They did pretty well, people were very unprepared, however my decks lost when the opponent refused to fight. As a result I tweaked the decks making them a little more able to stand by themselves, once everyone figured out the deck though I had to scrap it. Still, it worked pretty well and was quite fun. So basically, you take your idea and try to think of somethings to make it successful so it has some sort of edge and decent strategy, build it, then playtest and try to "mainstream" it a bit, hopefully removing some weaknesses and making it Tourney worthy.
The Anti-popular deck is a little more complicated. The first thing is figuring out what's popular, what you're bound to run into. Sometimes I even make a list, now don't get me wrong I'm about the most disorganized person you'll ever meet but I guess SWCCG means more to me than the real world or something ;o) I write the names of the 3-5 most popular decks for both sides. Then I name all the "magic bullet-esque" cards that deal well with those decks. After that I put a column that tells what deck tends to best against the deck in the first column. From there I look, it's rare to see something that will win against all "tech decks" however you should see a deck that will beat a few of them. I then look and see if I need a deck type or simply cards to beat the given deck. For example, some decks like MnT require more of a deck style to beat them then a few card combos. Where as decks like HBX simply require a group of cards. Using this knowledge I try to put together a deck style that will beat a few popular decks, ie: Y4 Occupation which will beat Revo and most MnT, along with Ops, then I add in "magic bullet" cards to give me a good, or even a fighting, chance against other decks. For example I add in some ships with Immunity, Security Precautions and Dengar in P1 in my Y4 Occ deck to give it a good shot against HBX.
Finally I look to see if it's a decent deck that can actually do something itself. Not just a jumble of anti-cards. If I see the deck does in fact have its own strategy, like Y4 Occ. which can drain, fight, and do damage with Rebel Bas Occ, then I know I have a decent deck. From there I go to the tweaking stage where I playtest it against the popular net-decks and make sure it performs admirably, I make changes as needed and them I'm ready to do some damage at a tourney.
Cheers,
Jon "Y2J" Schultz