I'm not as experienced in the CCG thing as half the guys here, so half of deckbuilding for me comes in the playing - if I play a couple games with a deck I generally have a good feeling for its strength/weakness, and that's where the single player thing is good - you can test against a variety of decks QUICKLY without having to handle the piss stress of battling ACE decks and DOUBIOUS connection issues.
While I'm playing, I keep things in mind, like:1) Am I getting too many/too little gold? Am I not able to launch half the memories in my hand before the game ends? Am I regularly running out of memories and then just waiting for my next card? This has to do with your income/cost ratio.
If you run out of cards regularly your deck could do with less resource boosters (cards which serve NO purpose other than to give income like Inheritance/Crops/Knowledge has a Price [crap card]).
If you can't play half your cards regularly then you need to adjust your costs down or play more boosters.
These are minor adjustments, 10 cards at a time is a lot. So tweak tweak tweak.
2) Am I able to push the same button enough to win? A deck must know how to win. Most decks are strong against one type of threat but strong against another. If a deck has a lot of anti-agent then their site destruction will be weak. Vice versa. Same with counters, discards, rush, control, etc. They all balance against something. If your deck tries too hard to do a lot of things then it'll be not very good at all things instead of being very good at one thing.
Your goal is to OVERLOAD one channel of threat. So FOCUS your deck.
3) Am I drawing the cards I want? 5 of everything crucial is important. Failing that, at least keep your deck down to 50 cards. Or thin out your deck with tutors or self-replacing cards like Tactical Upheaval, King's Order, Discreet Calling, Knowledge is Power, Family Heirloom, etc. The more chance you have to get to your key cards, the better.
Am I drawing too much of things I don't need? Sometimes you can sit with 3 RIs and know that your opponent is countering. You wish you had your Cesare. So you should have played less of RIs and more of Cesares (or King's Orders). You CAN have too much of a good thing.
4) Am I constantly failing to the same things? Addressing your deck's weakness could also work in your favour. If you lose a lot to sites (even 2 influence little damn sites), then maybe you should pack some anti-site. (Role Reversal, Mob Justice, Venezia San Polo, etc) If you lose to counters, then you need more threats, etc.
One way of overcoming those weaknesses is also to focus so much in your main threat channel that your weakness doesn't matter - you outpace your weakness.
5) Am I playing cards that my opponent need to deal with? A good measure is to play against a counter-heavy deck and see if you can out-threat your opponent. Every single card you play should have him want to counter it. Or at least as many as possible.
These are just some of the points to consider. The perfect deck, in my mind, will win consistently with very little cards in hand, and with opponent owning one region, because those are things you don't have to worry about when you can take the other two.
My Clothes Makes the Man variations haven't been super successful yet
Lately, I have to admit, I've become a tad lazy and try more to see what Bronto is up to, copy the deck as best as I can, play, tweak, and come up with my own variation. Learning from other people is also important - it'll teach you which cards are good together, how many is good, and seeing them in action drives it into your head.
I think there are more to this, I'll write more later maybe