I got a sleep therapist in - who showed us what to do - and I'm afraid it was not pleasant to hear my child cry - but I have to admit, it DID work. And thats tough love.
Gently go through the bedtime routine.
Put child to bed and kiss goodnight.
Leave the room.
Child will get up/cry for you.
Leave child to cry for 2 mins (1min for each year of the childs age)
Go in and settle child, without switching on lights or talking to her (do NOT engage in bartering)
Leave the room.
Child will get up/cry for you.
Leave child now for 4 mins (double the last time)
Go in and settle child without lights or talking.
Repeat this, always extending the amount of time between going in and settling the child.
Repeat this for AT LEAST 5 nights - both daddy and mommy must do this, take turns each night - so not one parent is seen as the ally.
Many will think this as harsh - after 3 years of no sleep, I eventually resorted to this method, and havn't looked back!