I just lost the game on ironman mode (AGAIN) because of an unfortunate encounter with reapers. I knew they were on the map, and had planned for them down to the minutest detail. I patiently waited for them to come to me as they mindlessly slaughtered all the civilians, but I had my team in position, covering every angle.
Unfortunately, I forgot to check around a certain corner (well actually I *had* checked it a couple turns prior) and one of them came around in range of my shield. Unfortunately he was equipped with a grenade, not a gun, so he couldn't react, and the shotgunner behind him whiffed 3 shots in a row. After he died it caused a chain reaction which eventually turned my entire squad into a bunch of mindless zombies. Such a ruthless game, I love it!
Unfortunately I seem to be losing the Geoscape battle. Not enough cash, scientists, radar, aircraft etc.
Read the quickstart guide. The key is building more radars. You want 2-3 major bases with 3 aircraft in each. Shooting down the UFOs on the geoscape map is what keeps your cashflow high so you have to maintain wide map coverage with as few bases as possible to keep gaining money.