You may already know but here is a video of it, it's very easy to do and there is no skill involved: Super Mario 3d Land Unlimited Life Trick - YouTube

Once you get 1,000 lives you get 1 crown, then 1,100 you get your second crown, and then 1,110 (the max) you get all 3 crowns and little mario has a hat and big mario is just hair which is opposite of normal.

(You will probably have to do this twice unless you have like 400+ lives already)