Yep, we had DS1 who did the 40 min naps in his first year. Drives you nuts, doesnt it? Sleep school helped his night sleeps, but he was a reflux baby and just had to outgrow the day sleeps (no matter how hard I tried to re-settle him). With DD this year (4.5months old) I've just got her into a good day sleeping routine. I ended up buying Sleep Right, Sleep Tight off ebay with DS2 and have been reading it again lately. She got into a sleep association of being fed to sleep and then when they come to the end of a sleep cycle, they will want the same thing that got them off to sleep initially (boob/dummy/being held etc) and if it's not present, then they wake up instead of rolling into the next sleep cycle. Sooo, not being able to slip into her room and put a boob back in her mouth after 40 mins, she tended to wake up quite a bit.

I've found what's helped the most was recognising her early sleep cues (mainly jerky arms, hands behind her ear/s, moving head to the side and avoiding eye contact) and then doing the wind down time by either lying her on my bed or the change table and softly singing the same song to her, before putting her into her bassinet.
We're also doing the 'Feed, play, sleep' cycle, so she gets a feed as soon as she's up, then play time then sleep time and I've stopped putting her on the boob before she sleeps. Initially I had to sit next to her and shhh and gently pat her off to sleep and now I can place her in her bed and leave the room and probably 8 out of 10 times, she'll go off, which means there's no longer a sleep association that she's relying on when she wakes after 40 mins. However, if I miss the early tired cues and she starts to yawn or cry, foget it. She's impossible to get down. It took lots and lots of repetition and a solid week or two at home to get there and I can now hear her on the monitor with a startled cry and a grissle at 40 mins and then silence again and she usually can sleep for another hour or two afterwards. Magic!!!