Normally you'd expect to give turret the longest line of sight you can, so they can fire longer at the closing enemey.  However this tactic doesn't work for alien turrets for 3 reasons:
1) They are slow firing, compared to frontiersmen weapons
2) They are very vulnerable to marine firepower
3) Slow moving projectiles can't hit a moving target at range.

However individual hits from alien turrets are much more powerful than the marine equivilent, so I have developed a stratergy to make best use of this situation.

Placing turrets on both sides of entrances, to catch marines off-gaurd as the come through the doorway.

===   ===
^          ^

where = is a wall and ^ is an offensive chamber.

This stratergy can be improved by adding webs across the doorway and defense chambers to regenerate the turrets.

= wall
^  turret
X  web

Variations:
1)
===   ===
^    XX    ^
2)
    ||
===XX
^

The most important thing is to not let the marines SEE even one turret until they're about 4 feet from an entire room full of them.  And keep in mind that turrets can't shoot through each other.

When webbing, a good tactic is to web from one section of the floor to another, and don't touch the walls, this way they are less visible. Don't forget to put more webs in a row so if the marines try and jump them they'll land on the webs anyways. If you see the marines using jetpacks you might want to just web up doorways in an obvious way that blocks all heights.

edit: I know this isn't directly on topic but I thought I'd tack it on anyways since I don't think much else can be said about turrets and I wouldn't start a new thread for a little detail like this.

that's only good (imho) on a forward outpost because on the main base they'd expect to see turrets...if they don't they know somethings up.

You'd be surprised how many people think that things do not exist until they see them... or get shot by them. Even at that, hiding something that the marines expect to find is better than placing it in clear view for them to spot as soon as they draw near. As said above, the first time a marine spots a turret should be when its made its first shot at you.

Id say you should always plan for the enemy to come and know exactly where everything is first. Relying on surprise only is a sure way to build a useless turret. If they are in groups then only one will die. If alone... respawn

Id place it based on TFC tactics really, swopping disp for webs. It should be easier to place as you can get to almost any surface, you have more than 1 web and the OC's have a 360... FOV? (if thats the term)

[Edit]It occurs to me that I have still not played the game , take what I said with a pinch of salt [/Edit]

I agree

Also, remember that a lone turret is a dead turret. Build clusters of at least three so they can kill a normal marine within acceptable time, and with luck distract marine fire from each other.

Put one defensive chamber nearby to heal them.

Absolutely, the turrets are actually pretty effective in the early game. A Defense Chamber or two does wonders to help keep them alive. It can also be a good place for aliens to retreat to to heal up. The have cover and something actively defending them.

In small hallways around corners are one of my favorite places for the OCs. Marines frequently run in groups and you can usually get one or two right off the bat, because they run around the corner, then can't go back because of the guys behind them.

-Slappy

Turrets can also be very, very good as a distraction.  If you set up a bunch of turrets near an ambush spot and leave one Skulk to hide nearby, it's pretty easy for that Skulk to demolish a marine squad while they're focusing on the turrets.

= Wall
+ Offense Chamber
* Defence Chamber
^ Web
X or x is null


xxxxxx=    =xxxxx
======    ======
     *+  ^^^^  +*

so you have 1 defensive chamber healing 1 offensive chamber, and the webs to catch marines.

main thing that all players wants to know: is there any use after that point of game where marines get grenadelauchers with offturrets?At least to me it seems that with few clips of grenades you can destroy whole alien defence...(and half base in same time too) :/

I guess I'm boring everyone by repeating this over and over, but normally, nadelaunchers are way too costly to afford more than one, which somehow limits the problem by itself.
The current resource issues have of course thrown this out of balance - it'll be fixed with the upcoming patch.

= wall
+ Offence Chamber
* Defence chamber
# Web

   =  =
=====##======
*+  ####   +*


    ++         <- These are for attraction

I'd suggest placing up to two more offensive turrets near the defchambers if you can afford it - higher firepower is always nice

I wanna give something to think about you: me and my friends are playing several matches in day at our private server and most effective strategy with aliens were this:

one JUST one evolve to gorg and start to find few resource places.in that while others go with sulks and start to parasite and kill and tease enemy marines with attack waves and dont let them to go resources.After that gorg have got some resource (like 3-4) he rush to second hive and take that... and then third.Main thing is what i want to say that dont focus too much on defence!With kinda rush aliens have much better changes to win at least in my experience...
agen bit offtopic but well...at least you all got something on your mind.Try this and find out do you feel it nice strategy too...

Also, when placing offensive chambers on both sides of a doorway, jump up and STACK the offensive chambers, twice the firepower.