The best thing I can recommend is to use sleep cues. I sing the same song (Twinkle twinkle) and then say "it's time for sleep darling, Mummy loves you" every time I put both my little ones to sleep. The only difference is for my DS I add "I'll see you in a couple of hours" before his day sleep and "I'll see you in the morning" before his bed time at night. They quickly learn to associate the song and the sleep time message with sleeping.