Some standard practices i follow when building FOBs:
First, i look at my FOB handy chart. which i upload on imgur.
I ask myself,
A: Whats the most marines i can get to crossfire on the smallest number of enemies?
Rather than build barricades RIGHT into the doorway, build a "U" shape around the inside of the door. Its like a math equation, you want the maximum amount of surface (i.e marines), concentrated on the least area (I.e Enemies).
B: Can i double layer my defenses? (Like a water tight boat)
Rather than just one layer, i try to make 2 layers. If the outer layer is damaged, it can be repaired by an engineer without breaking the inner safety bubble. If the 1st layer falls, engineers can build band-aid barricades around the wounded FOB section. You never build more than 2 layers, because its a waste of resources, and 3+ barricades cause bullets to IMMEDIATELY miss 50%+ more. (and engineers cant reach that far, dont build 3 barricades in a row)
C,D: Can i build in Multi-layers/ multi-section FOBS? (like a water tight boat again, but this time, with water tight compartments added as well)
Rather than just a single or double wall around a massive FOB area, i like to build mini sections that bi-sect the area into sections This is the ONLY way to get more than 2 barricades into a FOB defensive line. Essentially, your building rooms into your FOB, giving it more structure to withstand attacks from all sides, rather than a hollow ballon that pops as soon as its skin is ruptured (instead of losing air, you lose marines).
E: Can i place repairable barricades in front of non-repairable?
Rather than randomly placing metal and sandbag barricades around, i place the "REPAIRABLE" barricade in front of the one that you "CANT" fix. This allows me to use the metal barricades for their best ability, being repaired, and the sandbags for their best ability of being easy to move around in case holes need to be plugged. Use your materials to the best ability.
Build to the size of your force (i.e, if you expect to have 10 marines survive out of a force of 60, build to that number, not 60. less is more)
Other tips n tricks.
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Dont be afraid to remove and move barricades/ mines/ sentry guns around as the battle goes on ( if your gonna lose the front lines, theres no point in having a really strong rear line)(if you have a minefield, move it to the front lines, rather than let it waste points in the back lines)
Where do i put Machine guns/ Sentry guns? You look to see if you can get your machine placed on the small bottom section of the "L" in a crossfire. This allows the machine gun to "Rake" fire accross the front of your barricades. The machine gun/Sentry gun are your anchor points for a defensive line, and they allow you to "Rake off" any aliens who try to break through the long side of the barricades. Perfably, the machine gun anchor point should be in a "Defensive pocket". I.e that bottom "L" shape should be a reinfoced pillbox, or the enemy should only be able to attack the machine gun if they get in range of the marines on the big line portion of the "L". Either the enemy forces an attack on a tiny reinforced section, or they force the attack on a large section, but are shot to hell by the machine gun.
Finally, if you have spare resources, and you "Really need to use em", build funnel walls. sand bags placed horizontally to your front line instead of vertically. (the shape of a "]---" rather than "] ] ]") It allows your troops to fire outside, and acts as funnels to prevent side to side movement for enemies attacking your front, causing them to bump into one another or other stupid mistakes.