Just some simple things that can be applied to just about any system or genre.
A lot of these things are probably ideas that you already incorporate into your games (especially if you are a story-gamer or a "non-traditional" gamer), but I know that there are quite a few groups who will find these ideas new.
The first idea was something that I picked up in a forum a few weeks ago, I don't remember which forum it was in, or what game it was suggested for, but I was really tempted to throw it into my rewrite of FUBAR. I didn't, and instead I'll be incorporating it into one of the genre expansions for the game (maybe one focused around action movies)...
...anyway, enough with the background for this post...on to the meat...
A few of the old games I remember playing had a distinct problem with characters, it might have been something to do with the players (we were teenagers with few social outlets other than our gaming), but then again it might have been something to do with the fact that none of our games provided explicit instructions on how to "play" a character.
I think it was when I engaged in live roleplaying that I really picked up the idea of personifying a character by indulging in stereotypes, or really playing up certain character aspects. Before that, the games were all about numbers on a page, rolling dice and determining who'd end up with the biggest share of XP or treasure.
Going back to tabletop play, I saw a lot of players who still thought in this mind set, but who thought they were missing out on something.
This idea doesn't really push a traditional gamer into exotic territory like "shared narrative" or "GM-less play", it's more like an expansion of the concept of alignment.
When a player devises their character, they should add a single extra step into the process...simply identify which actor you could see playing the character you have in mind. Then play the character with the mannerisms and stereotypes common to that actor...
How would this character be played by Arnold Swarzenegger? John Cusack? Edward Norton? Keanu Reeves? Jack Black? What about applying the exact same set of attributes and skils to a character being portrayed by Angelina Jolie? Judy Dench?
It's just an interesting idea for getting into a mind-set.
It's worked for me in live games, and I've brought it to the table successfully on many occasions.