...Personally, I prefer a tower defense play-style with a few turrets (say 4 turrets in a colony of 12 guys). It is not a wise play style with the current solar flare mechanic...
Some researchable technology to prevent or warn of orbital raider drops right on your head (e.g. a no drop zone or a drop warning radar). Even if you totally dig into the mountain, you have to go outside for trading. Every once in a while there is an orbital drop of twice your number in bad guys right on top of your head with no warning. Any colonists outside just die right away. There is no mechanic or smart way of play to save them (at least that I have found; maybe I am missing something and just being clueless). This seems a bit too arbitrary to me. I hate having to load a saved game (feels like cheating) just because half my guys decided to step outside for the wrong hour. Now, I don�t mind the horde from the edge of the map that overwhelms me. That just means my defenses sucked, I needed better choke points, etc. But, just having them drop on my head with no chance for half my colony to survive seems wrong somehow.
A bit off-topic, but I've used the opposite strategy to fairly good effect: Build a ridiculous amount of turrets (with sandbag protection). You see, more raiders spawn when you have turrets because turrets slow raiders down significantly. While they're busy firing at the turret, you can flank them, slim their numbers from afar, etc. If you build many turrets, the attacking force is essentially deadlocked until they've destroyed all of the turrets or you've taken things up-close-and-personal.
Because I built so many turrets, raiders jumping in on top of the colony are at a disadvantage because they're completely surrounded by guns before my colonists even act. In my experience, even with the significant numbers disadvantage, the tactical advantage given by the turrets splinters the raiding group fairly quickly, which makes the attack much more manageable. Stick to the M-24 sniper rifle, miniguns, and (for close range affairs) either the R4 charge rifle or LMG. (If you can't get those, try for M16s and Uzis.) The turrets will handle the rest. The downside is, naturally, that you'll be rebuilding your turrets and sandbags fairly often.
If you don't have/want turrets, the best you can do (in my experience) is break your colony into sections where the colonists can retreat to if they're in danger. Beware large open spaces. Even in a worst-case scenario, it's better to have your base burned to the ground than a genocide. /offtopic