First, if there's anything already deployed there - a doombringer mine, a tech turret, whatever - don't put a claymore right by it. Don't want people clearing two mines with one shot now, right?
With that in mind, I'll show you my basic flag defence setups...
Arx Novena:
Side-to-side routes aren't terribly common, but they can be hard for the heavy to spot, so blocking them with mines or forcefields is common. Those guys don't get me a ton of kills, but they catch a surprising number of unwary raiders and infiltrators. I like to put them a few feet shy of the far doors on each side.
Those guys are the big money. They sit on the little slanted bit of the roof, right above the Blood Eagle symbol. The front one gets destroyed a fair bit, but it still gets quite a few hits. The back one? The back one is a total boss.
Crossfire:
I hate Crossfire, and I rarely play doombringer on it. Mines on the stand can work, but they have extremely short lifespans. Something like this...
... might catch some people, but there's a good chance it'll just get blown up. Now, what you should
definitely do is this:
That is the great redeeming feature of Crossfire.
Some people like to put mines behind the boxes (infiltrators like to hide there to regen) or in the doorways that lead to the generator area.
Drydock:
I also hate Drydock.
The mine on the right in the first picture is by far the best spot that I've found, but... flagstand defence on Drydock is pretty hobbled to begin with.
Katabatic:
The mine on the ramp should be up a little higher.
From the Blood Eagle perspective, the arrangement is different. But... I don't know that side of things as well.
Temple Ruins:
Doesn't get many kills, but it screws some people up.
Honestly, juggernauts wreck my face on this map most of the time.
Raindance:
The stand is so tiny there's barely any room for mines. Put them wherever you want. I recommend deploying the drop jammer either in the little room behind the flag or underneath the catwalk in the generator room.
Keep in mind that claymores are generally more visible than doombringer mines, as well.
EDIT: Oh, and Sunstar. I don't have pictures for Sunstar. Flag routes are pretty obvious, but if you have a doombringer, he should easily have that covered. It's not on the same level as Crossfire, but deploying claymores at the top of a conduit (the blue energy regen thingies) can be pretty good if you can predict which way the enemy will go once they hit the top.
EDIT2: All my images got broked. Had to re-upload.