Ah the little adventurers! My daughter did this at about 23 months. She spent about two weeks "practicing" - balancing over the rail of the crib and then one day I heard a thud in the morning and the crying. She was fine but definitely scared. We did a couple things.
First, we bought a video monitor. Got it used on Cragislist so it was 90$ and it's AWESOME!! SO now I can see exactly what she's doing, and go in there if she's in danger.
Second, we actually spent some time talking with her. We told her many times how she could get hurt climbing out of the crib, and that if she really needed out, she should call for Mommy or Grandma or whoever is watching her. Now you wouldn't think a 2 year old would "get" this concept - but she did. Now she calls for me in the morning or after her nap when she's ready to come out, plus we have the video monitor to keep tabs on her. She's almost 26 months so we've managed an extra 3 months this way, which is great for us as her room is not baby proof and will be impossible to do so.
THird, we bought the toddler rail for her crib so when she's ready to transition we are. I hope it's a long ways away yet!
We did buy one for the crib tents but it don't fit on her crib.
Good luck!