Use The Process: CAP -- TAP -- MAP (this is how i remember it in the correct order).
Now before telling you the process, you must understand this is something that has to be done constantly, NOT every minute or 30 seconds, BUT between most of your actions and most definitely when you are idle. This is the main mechanic of macro; to be constantly building whatever is available to you, with the given time, without the need of queuing. :D
Applying The Mental Process
CAP -- Pop CAP/Income -- Info Gathering/Building Macro -- Eye Location: Top-Right
This is merely your first action that you will be doing in the cycle, the entire purpose of this is to keep yourself up to date on your population cap and income, and therefore be able to base decisions on what to spend your money on (supply/units etc). This is also where you BUILD STRUCTURES/SUPPLY if necessary. This may sound self explanatory, but it is merely the process of constantly looking at you CAP and building SUPPLY that ensures good macro and keeps your money down. and you'd be surprised how often you don't look at your CAP... does "build more supply" sound familiar

TAP -- "The Tap" -- The Unit Macro -- Eye Location: Bottom-Centre
So you may be asking what the hell is "The Tap", well ill tell you... Just make sure that you are competent with hot keys and that you are selecting buildings via hot key NOT clicking them

1. Command Centre (Nexus/Queens)
2. Barracks (Gateway/Hatch)
3. Factory (Robotics/Lair)
4. Starport (Stargate/Hive)
5. Engineering Bay (Forge/Evolution Chamber)
Now "The Tap" is merely the process of taping each number hot key and LOOKING at the bottom of the screen to see the building's current status and deciding there and then whether anything needs to be built, then moving on to the next hot key. This ensures you know the current status of all your buildings and are constantly building from your production structures, here is an example of a cycle of TAP using the designated hot keys:-
TAP "1" -- Command Centre -- SCV at 80% -- Build SCV -- TAP "S"
TAP "2" -- Barracks -- No Production -- Build Marine -- TAP "A"
TAP "3" -- Factory -- Siege Tank at 30% -- **Skip**
TAP "4" -- Starport -- Reactor Still Building -- **Skip**
TAP "5" -- Engineering Bay -- Just Finished Building -- Upgrade Weapons -- TAP "E"
It is as simple as that. However, it takes quite a bit of practice to implement since you will keep forgetting at first, but forcing yourself to do it again and again will make it pretty much an automatic reaction, which is what you want when you begin to start focusing on strategies. The only problem is that this relies on the fact that you should constantly be hot keying buildings/units, again this is a learned process that will become more natural in time, but you will also find as you initiate the TAP that you will notice something is missing and then add the appropriate hot key, the more things you have hot keyed in your TAP cycle the better. Finally and most importantly it allows you to macro while NOT focusing on your base, allowing you to macro in battle or elsewhere you are looking & remember it's not set in stone, there is no use cycling through buildings if they are redundant (i.e the factory in marine/medivac)
MAP -- MiniMap -- Info Gathering/Army Decisions -- Eye Location: Bottom-Left
The minimap not exactly key to macro, but key to VICTORY. This screen provides you the most IMPORTANT information available to you and gives you the means to travel anywhere on the map via instantaneous click. It lets you know where your troops are, and where their troops are. What decisions you should make and so on... It is critical that you spend MORE time looking at the minimap than the main screen. But why is the mini map MORE IMPORTANT than the main screen? :-
When not in battle
1. The minimap shows you everything that is going on via the combined sight radius of ALL your structures and units.
2. It shows you an enemy army before they attack your base (yes, that's right folks

3. The minimap is a reminder to expand, if you look and see you have enough minerals an you know you have an advantage already over your opponent... why not?
4. When Zerg the minimap is your BEST indication as to whether to morph drones or not!
When in battle
1. The minimap allows you to co-ordinate your forces and see where they are positioned throughout the map (A good way to pick up stray forces that were neglected to be hot keyed)
2. It shows you if there are multiple points of attack occurring (i.e reapers/mutas at back of base). It is so IMPORTANT that you look at your minimap during a battle, cause if you don't nothing will be there when you return home

3. If you are on the attack it also ensures your reinforcements are not being intercepted
Final Thoughts
So really the mind set you got to have is to always be in macro mode, constantly producing units, and being aware of your money/supply. From applying the process you will come to notice that you are making a clockwise motion with your eyes as you go from CAP to TAP to MAP and over time this becomes natural, you will realise that you already know what your supply is because your eyes have constantly being traversing the screen without you thinking about it. Essentially once you break the habit of staring directly at the centre of the screen, you will become more engaged with the game and start enjoying yourself a bit more. Hope This Helps
