There are a few things i try to remember whilst playing ELO.
1) You don't have to ko to win. Sometimes you might think if i fury with this card then i might be able to ko but if you don't fury you have a higher chance of your card getting through and if you lower your life points lower then your opponents you win. (if you have to ko to win though go for the ko).
2) You don't get extra points for winning quickly so think your move through.
Apart from cards and pillz and stuff, theres always that psychological thing. I personally tend to become *really* nervous when i climb into the higher ranks (yeah, some 1300+ is quite a number for me since im not that active in UR). Ergo, i usually make more stupid mistakes, miscalculate, overlook opponent cards' abilities and all that. It causes me to lose several matches in a row untill i fall back down to my lower bracket 'zone of comfort'.
1200 seems to be the most important number when playing 'casual' ELO, since its the borderline that defines if u will get a considerable reward or not. So maybe u just get a bit excited at that point. Much like i do
Try and keep calm, that simple. If you feel like getting uncomfortable, take a short break. Unless u're like hardcore DT player, theres usually no rush. Go smoke a cigarette or smth (not that i set u on to start smoking at all, lol, just in case u do). Helps me a lot.
One more good thing is not to be concerned with ranks. Its very easy to overlook your score (at least via the new interface) and this actually turned out to help me quite a bit Just keep playing and dont pay attention to those little figures that pop up between matches. Its just a game, have your fun.
1st) Proper deck (many people has this issue, read more.. theory is for your reference, most importantly is practical)
2nd) Experiences (comes from reading other clans' card during your free time, run through which cards are good / bad against certain different cards)
3rd) Getting used to your deck (Play more, form a proper deck and keep practising, don't be fickle and change too often else it will mess up with your mind) *Note* Although that doesn't apply to everybody
Play consistently, as in do not give up easily or rush the game, over-pilling 1st match doesn't mean you will definitely lose the match.