I too, have a 2.5 year old daughter who has never been a great sleeper, unless she was sleeping with one of us, and never could just put her down and have her fall asleep on her own. (Neither is my DS, who is just about to be 6 months old. My mom tells me I was one of those kids who could do that - but apparently I was not destined to have one of those! I blame my husband's genes! =) )
Like newmamalizzy, in the past few months, I too have come to the conclusion that my DH & I are the limiting factor, at least in part. If we're in the room, it seems to take much longer for her to fall asleep. Somewhere along the line I decided to try the leave and come back to check thing, after we've done the whole bedtime routine - books, singing, nursing. I don't mind lying with her for some period of time, but do long to have evenings to do grown-up things again, even if those grown-up things are laundry/dishes - or watching mindless TV! Plus, my DS also has a bad habit of not staying asleep for long stretches until I go to bed. So, actually, I used him as an excuse when I started - I would have him w/ us in DD's bed, and I'd get up to go put him down, which often turns into him waking up unless the stars are aligned correctly. And then I'd come back to check on her periodically til she was asleep. That worked pretty well for awhile, though on occasion she'd come out of her room looking for me - but would seem genuinely upset/frightened. (we moved not too long before this, so can't say as I really blame her...) I also tried to remind myself that you can't force someone to go to sleep, but I could at least make sure she was resting in bed. (not to say that she never gets out of bed, but most times she would stay put.)
In the past month or so, at naptime, I started doing the same thing - get her settled in bed w/ a shortened routine and then I'd turn on her CD that she listens to to go to sleep AND I'd set my cell phone timer & tell her she had to stay in bed until the timer went off. She could read or sing or play quietly with her stuffed animals, but she had to stay in bed. Hug, kiss, "have a good nap" - and I'd leave the room, and that has been working really well. If the timer happens to go off before she falls asleep, she can get up - we all have times when we can't fall asleep even when we're tired after all, but that has happened once. It works so well I actually went and bought a cheap kitchen timer at the dollar store. She loves that thing. "Where's my timer?" I set it for an hour and leave, and most of the time, I just go in the other room, b/c I try (soo soo hard!) to coordinate nap times, and leave to take DS into my room which is right next to hers to settle him in for what is usually a good long nap, thank goodness! By the time I get him settled into his crib or my bed, she's out. The trick is to make sure I get back in there before the timer goes off, lest it wake her.
We've started using the timer at night too, though it could backfire if she did stay awake til it went off, b/c clearly we don't want her getting up that much after she should be asleep! So far, so good though. Not to say that she doesn't sometimes wake up looking for one of us, and she pretty near always ends up in bed with us sometime in the middle of the night, or early early morning if we're lucky - but at least we get some sleep w/ my DH & I in the same bed - if she comes into our room, he either takes her back to her room and falls asleep making sure she's asleep, or lets her in our bed & goes to sleep in the other room. Musical beds - not our preference, but we do all get some sleep that way. (If I'd known we'd end up co-sleeping, I would have gotten the king sized mattress DH wanted in the first place - would've been much more comfortable!!)
The other thing we're working on w/ her that has helped me maintain some sense of sanity is trying to move her afternoon nap earlier. The No-Cry sleep books have a handy chart that lists average amounts of "endurable" awake hours for kids of different ages... and for 2-3 year olds, they can stand to be awake about 5-6.5 hours at a stretch before they need a nap (of course, not all kids will match that, but seems to be true for her). So, if she naps from 2-4, then it's not real likely she's going to be able to go to sleep by 7:30 or 8 PM, which I would LOVE! And paying better attention to her, I have realized she's ready for her nap (or at least ready to head to bed for some down time before falling asleep) by 12 or 12:30...I'm shooting to get her asleep by 1 so that we can have the earlier bedtime we want and that she needs, b/c she doesn't sleep in just because she's gone to bed later. When I can make that happen, it really helps a lot. When I can't, then at least I know not to expect her to go to sleep that early. She still starts the wind down pretty early though - I have her come upstairs w/ me while I get DS ready for bed and then I nurse him to sleep while reading to her. She ends up w/ a lot of stories that way before nursing, and then when she seems ready, I leave w/ the timer on, etc. (DH often does bedtime w/ her when he's home at night, but he has a lot of evening obligations, so at least 1/2 of the week, I'm on my own.)
I'm with all you other mamas. It sucks to have less-than-good sleepers, esp. when we all know some other families w/ kiddos who sleep so well, so young, and we can start to question what we are doing wrong (at least I do!). And I do try to do what I can do to help move 'em closer to better, longer sleep, not just because I need it, but because their little bodies also need it, but I know it's not just me; a lot of it has to do with the kids themselves...
hang in there!!