The Frontiersmen have four types of buildings to consider:
1) Vital buildings - CC, Infantry Portal
2) Commander-use buildings - Arms Lab, Observatory
3) Turret factorys and turrets.
4) Special-use buildings - Armory, Prototype Lab, and Phase Gate.
Each building type has an optimal positioning in the base and other map areas. A general strategy to follow is to place most buildings (exceptions include the IP and Phase Gate) against walls if possible, to prevent aliens from hiding behind them.
1) High-use buildings need to be fairly out-in the open, but protected if possible. To best protect them, be sure to place turrets on BOTH SIDES of the Command Center. I frequently place the CC, IP, and two turrets in a diamond pattern (CC and IP on opposite corners), so the turrets protect each other and the other buildings as well. This is completely sufficient for defense until the aliens gain Fades.
2) Buildings like the Arms Lab and Observatory never need to be accessed by marines. Therefore, place them out of the way, tucked in a corner of the base. Again, place them against a wall if possible.
3) Turret Factories should be placed against a wall, and remember that the factory doesn't have to be next to where you plan to place a turret - simply nearby. It may be possible to build a factory against a wall of a secure room to build turrets in the adjoining corridor! If your are upgrading a factory to siege, be sure to have marines or turrets to defend it.
If you are building a small outpost, it may be economical to recycle the factory once you have placed turrets. But remember that if those turrets go down, you can't replace them.
Two things to remember about turrets. #1, remember that they must have true line of sight. Place buildings between turrets or against a wall. Placing turrets against walls is a possible strategy, but its benefits (alien cannot circle strafe around the turret) are balanced by its faults (turret still attempts to track 360 degrees).
Secondly, a good rule of thumb is the following: make sure that if an alien can see one turret, TWO turrets can see him. Place turrets at corners in an angled pattern so the both cover approximately the same angle. However, be careful to not put them directly in line, as they cannot shoot through each other (though they will not damage each other).
4) These buildings don't have simple rules for placement, but here are my opinions on each. In all cases, make sure they're protected by turrets at least on one side!
Armory: place fairly near the Infantry Portal. Not generally an alien target, so it's not terribly necessary to place against a wall. However, keep in mind that your marines are vulnerable while reloading, as their view obscured by the armory. Don't place it where an alien can snipe your troops!
Prototype Lab: place this out of the way, probably against a wall; jetpacks and HA are infrequently handed out, so it doesn't have to be terribly convenient.
Phase Gate: A gate in the marine base should have at least two turrets with clear line of sight to it; expect the aliens to use the gate if they can. A gate in the field should DEFINITELY be protected by two or three turrets; they are high-priority targets for defensive aliens. Note: turrets can shoot over a Phase Gate.
I may revisit this with screenshots, especially of turret placement... happy building!!