For AD - usually place them on Screen, so they could move back if suddenly approached by the ground units. Also (mostly relevant to the NATO AD besides Gepard) try to position them so they won't be firing at helos from their max distance, since that way they only spot themselves, but do not deal much damage. Mostly I try to lure helos closer, so the AD fire will be more effective. Soviet AD is, in my opinion, better in that regard (especially SA-11 and SA-19), and I try to place them on the highround in my rear, so they'll have bigger area of fire.
As for Helos - it seems that they don't suffer from arty fire that much, so if it is long range ATGM Helos, I try to put them on Screen on the good firing highground position to destroy enemy armor. As for Helos with unguided missiles, I send them close to the enemy, so they can fire their ammo with most effectivenes until they are empty. Sometimes it pays off, sometimes turns to be costly. But both of methods are used if opponent don't have heavy AD, in this case i firstly try to locate AD with recon, or 1 Helo section, then to suppress or destroy it with arty. Also in the latter case I prefer to send Helos to the enmy rear more often.