What worked for me as a child was the idea that if I didn't practice I was letting my teacher down.
Not quite as doom and gloom as it seems! I had music lessons, and other similar one-to-one tuition elsewhere. My parents never hid the cost, but nor did they hold it over my head. What they did instead was point out that my teachers were 'experts' in what they were doing, and that I was bloody lucky that they were willing to teach me- a complete nobody and a novice in their eyes. There was plenty of other people who would like to learn what I was learning, and if I didn't give my teacher the respect of practicing, then it would be my teacher who would cancel the lessons to allow in a more hard-working, worthy student.
I've no idea whether they really would have 'sacked me off' or not 
, but the threat alone when I was younger was enough to make me take practice and lessons a little more seriously. I wasn't just responsible for myself, I was responsible to somebody 'better than me' who had taken the time to teach me. It was more 'I'm the lucky one to have tuition, and my teacher will be glad to have me as a student when she hears how fluent this piece is this week' rather than 'lessons costs some amount of money I don't understand because I'm eleven, and school days are so long and tiring at secondary, this is hard!'. Of course, that requires the understanding that your teacher can, indeed, tell very clearly when you have not bothered to practice, but that it's also safe to say "I've tried, I can't get this at all, and I can't articulate why, so tell me."
All that rambling ^ but also, god yes, the easier to just sit down/pick up and play the more likely you are to do it. The playing is the fun part, all the prep is just a barrier that makes you go 'meh, later'.