Careful observation, reading these articles, playing other trainers...
I've noticed something common in many trainers. Often times, people will play
with what they think is strongest, not with their most favorite. At times,
I've have more trouble against an opponent's Parasect than another player's
"Misty's Seadra", simply because the Parasect was played with more heart and
cunning. There's potential in any card, don't just pay attention to that
little symbol in the bottom-right corner.
    Anyhows, personal card tips eh? Here's one for you... You can never have
too many trainers, those are the key's to my victory. I'm a fan of Dark
pokemon; rocket that is, not darkness element. They don't cause overly much
in the way of damage but, coupled with good trainers, I've managed to win
most battles with perhaps one prize lost. I'm not bragging and to prove it
I'll tell you how. Have a clearly defined defense and offense strategy.
Example: my offensive strategy is to always be doing something. I'm a deck
burner who makes the most of every turn. My number one card for this is
Professor Oak. I don't care how many cards I tear through, because my
opponent won't keep up. I also like damage in any form. Wether with trainers
or poison, or even my gym(I like mines). My defensive strategy? Well, ok... I
don't let them attack. After hitting their defending pokemon early, they
often power up a back pokemon. I have as many super energy removals as
allowed to soften up those back pokemon. This is even better... Take the
energy from the front pokemon and pummel the helpless pokemon on the bench. I
leave that task to my "Dark Arbok" and "Dark Golbat". Yep, poison is one of
my most favorite elements. Well anyways, these are only examples. Plan ahead
and play with your heart. Hmmm...(Hehe, that's why I'm one of the best rocket
trainers)