For my first playthrough in games I generally try to think of what I want to get done in the game, and pick something that I think will let me see the most content. As far as decisions go, I generally do what I think is "right", or occasionally what I think will get me the most reward. So it's generally a talky guy, with high intelligence, decent agility, and I take points from Endurance and Charisma to stick elsewhere. I tend to tag small guns, speech and then doctor or lockpicking in the first two games, and always lockpicking in the new ones. When I played Fallout 3 I picked small guns, lockpicking, and repair, because speech was mostly worthless unless you want to get more loot.
As far as traits go, in Fallout and Fallout 2 I go for small frame and finesse, in NV I go for small frame and usually Good Natured or the glasses one, which I can't remember the name of.
For subsequent plays, I occasionally mix things up by going out of my way to bump up skills and attributes I wouldn't normally use, and make decisions I wouldn't normally make. Like a high strength/endurance low intelligence character who focuses on unarmed combat named "NUNPUNCHER", or a pacifist playthrough (I've done one in Fallout, haven't tried it in New Vegas yet).
I'm not one of these people who makes up a backstory for his character and goes out and only does things based on that, but by about a quarter of the way through the game I generally have an idea of what kind of character I want to play as, and decisions tend to be fairly consistent in regards to that. But the imaginary role playing thing where, for example, you pretend that you're a raider so you only wear what raiders wear and only use weapons they would use in the game, or playing a character who loves teddy bears and just can't walk past one without picking up - that's not for me.
Incidentally, I do collect pencils in New Vegas. Because they're weightless and worthless and pretty much everywhere, and it's just sort of funny to me. There was a bit in OWB where I wasn't paying attention and all of my graphite pencils, through some form of alchemy, were turned into lead and scrap metal. I was genuinely horrified by this.