1 - Focus on the mechanics
1.1 - Macro
1.2 - Micro
2 - Focus on the build
2.1 - Goal
2.2 - Execution
2.3 - Hitting timings
3 - Focus on the Army
3.2 Army Count
4- Focus on the Opponent
4.1 - Build
4.2 - Hidden Tech
4.3 - Timings
You'll have to excuse me as I'm writing this at work. I'll probably go back and do some editing based on feedback.
1- Focus on the Mechanics
1.1 - Macro - In short Macro is your ability to control the flow and function of your base. This is the core of solid game play. Good macro is your core ability for winning games. That being said here's what you need to look for.
a) Are you making enough workers? Worker creation is a little bit different for each race, but a good benchmarks are
Terran: ~50 SCV's + mules at 10:00
Protoss: ~55 - 60 probes at 10:00
Zerg: ~55 Drones at 8:00
The bench marks assume you're trying to play a macro style standard game not 1 base all ins
b) Are you properly producing units?
Terran: No idle structures, the entire game; If you have units que'd you need additional production facilities. A good example is 1 rax fe, into 5 rax, 1 starport, 1 factory for marine tank. As your # of bases goes up to 3/4 you'll need to add additional facilities. Usually around 9-10 barracks 2 factories, 2 starports for MMM + Tank.
Protoss: Are you warping every time your gates are off cd until you are max? Are you chronoboosting robotics bay units? If you find you're floating minerals / gas after your round of warp, you'll need more gates or more robotics bays
Zerg: With each round of larvae you have to choose to make drones or offensive units. You should essentially be droning whenever possible and making the bare minimum number of fighters / static defenses until you are saturated at least on 2 bases. The big macro thing for zerg is injects, are you hitting every single inject on time? Are there any times where you queen injecting is idle and there are no additional larvae spawning? Do you have enough larva to use all of your pooled resources?
If you are constantly making workers/units with no gaps in time macro can easily carry you into plat/diamond level without much thought to composition or upgrades.
Micro - In short micro is the control of your army and individual units to make them more effective against enemy units.
Melee units - you should avoid chokes, and try to get surrounds, these units should always be out in front of your ranged units.
Ranged units vs other ranged units. Try to arc as best you can so that more of your units can be firing at once. An arc for all intents and purpose is the C shape you make with your units.
Ranged vs melee units, try to put your ranged units back against a wall or against a choke to funnel your opponents army. Are you microing whenever you're not macroing. Did you lose an engagement because your opponent arc'd and you didn't?
Every game you play, you should have a specific build order in mind. You should try and practice 1 build order vs each race, and do the same thing every time at least up to 10 minutes.
Each build order should have a specific goal. The ability to force your opponent into a play style you can handle or counter. The worst thing you can do is be 15 minutes into the game going hm... What should I build now? Having a combination of marines + vikings while your opponent goes Ultralisk's just isn't going to be very effective. There are a lot of builds out there, with a specific purpose. Do you have a build order in mind? What are the goals of that build and are you achieving those goals?
For example hellion openings against zerg are designed to do economic damage to zerg by either forcing them to waste larva on offensive units instead of droning or flat out killing drones. Are you letting your hellions get surrounded by lings and killed off cost ineffectively? Or are you running by up to the main and kill 15+ workers? Are you executing the build properly?
Each build will have specific timings for a specific reason. If you're late on building structures in that particular build order, your entire build will be late and overall much less effective. If you're doing a 4 warpgate all in, but you don't get your first proxy warp off until 8 minutes when it's supposed to be around 6:30 you're opponent will have too much defense in place and easily deflect.