I'm just emptying out my head into a post...

Sometimes players have difficulty roleplaying a sheet of paper - have they invested any significant amount of time in creating a back story for their characters? What about personality quirks? Accent of their spoken language? Motivations? Hopes? Dreams? I've always found that a little encouragement in building out a collection of stats into an alternate persona helps. Hell, you could even do something like 1-on-1 prologues w/ an experience point bonus for completion.

Speaking of experience points - players are often given bonuses for completing portions of major story arcs, coming up with an ingenious plan to thwart whatever trouble they come up against, etc. You can easily add in a bonus to whomever you feel was truest to their character/best role-played that day. A little healthy competition in the party shouldn't hurt - but it could go over the top if left completely unchecked.

Atmosphere can be important to some people - adding in fantasy styled background music to your games can influence people to get into character. Look to games like The Elder Scrolls, Neverwinter Nights, etc for inspiration if you go this route. Actually - I find that anything by Jeremy Soule is appropriate. Remember, different peoples are influenced by different mediums - immersion is extremely important to role-playing in my experience.

Ensure that your players understand that role-playing won't result in mocking - if you have a player or players in the group that actively engage in discouraging others from assuming their character - take them aside and express to them that this sort of thing is unacceptable. More to the point of promoting a space that's conducive to role-playing: remove distractions - Cell phones are the fucking devil. If you allow players to use laptops for their sheets/books - make sure they know that they need to be involved in the game even when it's not their turn.