My DD sounds exactly the same as yours, except then she became a terrible night sleeper as well. I just went to sleep school on Wednesday (day time only) and they suggested the following routine for 3-6 mths olds:
Wake say 7am: Feed then Play.
8.30-9.00 Bed/Sleep 1.5 - 2 hours (try for 2 hrs)
10.30-11.00 Feed then Play
12.30pm - 1.00p.m Bed/Sleep (1.5 - 2 hrs) try for at least 1.5 hrs
2.30 - 3.00 Feed and play
5pm - 7pm Bed/Sleep for approx 1 hour
7pm Feed, Bath, Quiet Time 8pm Bed
10-11 Rollover Feed then demand feed overnight but not under 4 hrs, aim for long stretch.
I varied their routine as my DD wakes approx 8am
Since reintroducing the Rollover Feed (we dropped this when she was fairly young) she has slept through the night.
They say to always wake bubs for feed during the day (approx 4 hourly) and them to sleep for approx 2 hours at a time during the day. I also vary this b/c it depends on what sort of day DD has had. They also suggest that you keep bubs in bed for the whole 2 hrs, even if they wake earlier and try to re-settle. This is suppose to encourage them to get used to sleeping that long.
I guess I have found being consitent and persistent is helping and not running to my DD's room as soon as she grumbles, I think this was stimulating her, not helping her go to sleep?? The other thing they say to do is not give eye contract, but I can't do that. I look at my DD, but try not to talk to her too much and pat her gently or stroke her face.
I've also just stopped wrapping DD and put her in sleeping bags cos the little Houdini kept wriggling out and was more unsettled. She also rolls over now, so it is not safe for her to be wrapped.
You could also try different positions for them to lie in. We have found now that DD sometimes likes to lie on her tummy (safe b/c she can move her head to the side to breathe) and she sometimes settles easier, but I always put her on her back first. You could try side positions during the day and roll up towels on either side of bubs so they don't face plough into the mattress if you feel it isn't safe if they roll on their tums.
The above are some tips, you may or may not want to use. Some people don't like 'routines' and I understand that, I tried not introducing a 'routine', letting DD lead us but it didn't work and it all got a bit out of hand.
By the way DD is 18 weeks old.