for our daughter (who just turned two) we put one of those pressure mounted gates just outside her doorway (about 2 feet)... she can open the door, but can't get any farther than that. now that she knows she can't go anywhere, it seems to keep her from leaving her room.
though in your case, she'd probably just cry at the gate... but you never know. regardless, when she gets up.. you need to take her back to bed. dont' say a word, don't yell or try to reason, just take her back and put her in bed. eventually she'll figure out you won't cave in. and do waht you are already doing, keep the bed time routine consistent.
btw... my sister used a timer for a while... as in, once her daughter was in bed and ready to go, she'd read her stories and sit with her and talk about her day, but when the timer went off, she would kiss her good-night, turn out the light and leave... somehow this worked really well, like the timer had to be obeyed, and it took the guilt out of leaving.
good luck!