My DS was the same as your DD - he would only sleep 40 mins at a time, resulting in not so good night sleeps. We use a CD on repeat, the same as Erin suggested and like Trish said, I would also time his sleeps so as soon as I heard tha first stirring at 40mins, would go in and pop his dummy back in. I also use a music box that plays a little melody for 10 mins and does a light show on the ceiling - I have found using this at the start of every sleep he has learnt to associate with sleep time, and when he stirs I can just pop it back on and he will usually drift back off to sleep. It took a while - some days letting him cry etc, or listening to him talk to himself for a good 20mins but eventually we managed to get past it and he is now a good day sleeper. At the age your DD is I think I could get 2 two hour sleeps and sometime a short nap in late afternoon. Another tip - we use a Jolly Jumper which I pop him into for about 15 mins before his day sleeps. All that bouncing around seems to tire him out! I am having a mummy brain day and can't remember what age we started that from - around the time that he had good head and neck control etc.

Good luck with it all! Here's hoping you get some lovely long day sleeps soon!