This seems a bit obvious, but uh..I'll throw it here anyways. Try to sneak up on enemies and always shoot while you're crouched. If there's something you really can't beat, get 2 followers and make sure they engage first while you sit back (crouched) and pick things off with your highest DAM weapons. Try to find something that has high DAM but still allows you to get off quite a few shots in VATS. I preferred the 10mm Submachine Gun. It has decent damage, fires very quickly, has plenty of ammo and I was able to get off atleast 4-5 squeezes in VATS. Also make sure you're not using the -wrong- ammo vs certain enemy types, this has a tendency to gimp you instead of making you stronger. I'm inclined to agree with Danku here, I can't really argue Fallout being too hard - especially if you have high Gun skill.
Worst case, use ED-E and try to avoid enemies you don't want to fight, ED-E's perk let's you see any NPC within a very wide radius. I've heard Melee is much better than Fallout 3 (not hard to accomplish) but Guns are the most powerful. This would go hand in hand with my experience.
That's like saying a Mario game is too easy, so if you wanted more of a challenge you should play with your feet.
It's more like saying if you think Mario is too easy, don't use mushrooms or any other things that effectively give you an extra 'hit' before death. But I'm not really a fan of saying "that's like saying...', it's much too easy to skew things to fit your argument.