Sectiods are girls

There are worse things than snipers. Mind control's been mentioned, but I've been sniped iinside my skyranger before disembarking, with blasters. That sux. Big time.
I think the proximity of the aliens to your landing zone may have something to do with your difficulty level. So if you keep getting wasted, try starting a new game on gumby mode.
Other advice just boils down to keeping your men in little groups that allow each group to cover different areas and moving slowly with the "reserve snap-shot" (or autoshot, or whatever) button selected.
Two to three teams of four work well. And if everyone has an auto-shot of time spare, when them nasties leap out from behind the shrubbery, four guys unloading autofire into the sucker will probably make the sucker think twice before doing it again.