My second son had little interest in cycling at all and it took a bit of effort to get him involved.
Make sure the bike is appealing to the child. Involving her in choosing the bike is a good first step but if the bike has already been bought (or brought by Santa), they can still be decorated with simple charms or stickers.
This is the perennial advice, which works for reinforcing any behaviour. Praise any success and doubly praise any failures which are overcome. For instance, it's hard to synchronise the use of the pedal, or even to keep your feet -on- the pedals, so initially make a big deal of those.
Cycle in nice locations
Whilst cycling up and down the garden path is all well and good, it's not particularly inspiring. Imagine if you were able to pedal around a local park or (as we did) the local zoo! It's not only a treat to go there but your daughter is doing so in a novel way. I'd definitely recommend this.
Picture the future
My son saw no benefit in cycling. He could walk to school and Daddy and mummy would drive him to far away places if he had to go there. So why cycle?
I told him all the cool things we -could- do, if only he could cycle. After all, we'd never get our little Mazda up a volcano but we -could- cycle up! Or we could camp in the woods and cycle across rope bridges!
That sort of thing. I'm hoping that he forgets about the volcano bit...