This is my second journey to masters. I made the first trip successfully in season 1, took about half a year off and ended up in plat this season.
Now I just made it to diamond again, and I feel obliged to give some tips on improving to lower level players like myself.
Please remember that this is written for lower level players! I've been in mid-masters before, and the focus of training does shift significantly.
1. Play consistently
Try to play a game everyday. Doesn't matter how much you play, doesn't matter if it's a custom or ladder or ai match. Play everyday if possible.
I have to emphasize this point because the biggest difference between the plat-me and masters-me is mechanics. For lower level players, our mechanics are horrible. No question.
Playing as much as possible is (obviously) the best way to improve your mechanics. But playing everyday, in my opinion, is the most important factor in ingraining it into muscle memory, even moreso than mass gaming on one particular day. If you want to develop a habit, you do that habit everyday, you don't do that habit 10000 times today. If you want to learn an instrument, you play abit everyday, you don't play a ton once every week. Of course, if you can play a crap ton everyday, that's the best i guess.
Still, everyone learns differently, so feel free to disagree.
2. Watch your replays
This makes obvious sense. Watch your replays to see where you went wrong. Emphasis on: where you when wrong. As day9 said in one of his dailies, at our level, it really doesn't matter what the opponent is doing. Every time you lose it's an error in your game plan, in your macro, in your scouting, in your mechanics. As long as you're playing well, your opponent can be map-hacking and you'll still win. At this level you can out-skill your opponent without out-thinking him, so don't focus on what your opponent is doing.
With that said, I would like to talk about how you should watch your replays. Again, don't overthink it and look for errors in your play. For example, in TvZ, if you lose to broodlords, don't say "I didn't build vikings in time, I need to build them earlier next time". Instead, look for the fundamental flaw in your play, say : " I need to keep an eye out for his greater spire", because the truth is, you didn't lose because of a poor unit composition or not having the proper counter, you lost because you didn't scout.
3. Mechanics
The most important aspect of your game. You can have the best knowledge of the game, but if you can't translate it into the game, it's worthless.
As mentioned before, the best way to improve your mechanics is to play consistently, imo. But there's another aspect of it, try. You need to be trying, pushing yourself outside your comfort zone in order to improve.
But what exactly should you be trying?
Here's what I focus on. You can do it differently, I really dunno what's best.
Be accurate. Take your time, and do things right the first time. Click once for everything if possible and reduce redundancy. Want to improve your APM? Then stop missing so you can do more shit more quickly.
Don't spam. Some people swear by spamming, saying it keeps them warm and fast. But it directly conflicts with being accurate and efficient. For the purposes of training, I would suggest not spamming, just because you won't want to tell your brain that having meaningless actions is okay. It's not.
4. Have a plan
Always go in with a plan. Just a simple early/mid/late game plan. The importance of this ties back to, surprise surprise, your mechanics! You don't want to think too much in a game. In most cases, at the lower levels (plat and below in this case), most people are making poor decisions anyway. It places you in weird situations where the opponent has made a horrible decision and you have to react. If you had a good game plan, a good decision was already made before the game and you'll most definitely come out ahead. It's extremely rare that an opponent makes a unorthodox good decision that requires you to change your plan at this level of play.
I've gone off-point. Basically, you want to have a good plan so you can concentrate on just carrying out the plan, thus dedicating all your effort into improving your mechanics.
5. Have a build order
Self explanatory, same as the previous point really. It's important to stick to it though. Two reasons. First, you might see a glaring weakness in your opponent's play that you want to exploit ( which requires you to deviate from your BO), but you shouldn't. Chances are, once you get promoted and start playing with better players, other people won't be making that mistake. So even if you win, you just wasted a game not practising your BO. Secondly, you want to consistently get into a mid-game with a situation that you are familiar with, so you can practice your game plan (and thus mechanics). If you picked a good BO, you'll get into the midgame in a fairly decent position anyway, so why take a risk by deviating from your plan?
As I wrote this, I started to realise that I'm just echoing day[9]'s various dailies. Sean you're awesome.
Also, all my points seem to be saying the same thing in a different way: "Improve your mechanics". That's because in both my attempts to climb the ladder, my mechanics seem to have the greatest impact on my performance.
Hope this helps someone out there.
YMMV of course
tldr; Improve your mechanics.