I accidentally over-watered my last batch of terrarium plants and subsequently sent them to a watery grave :) But, the good news is that I kill plants much less often than I used to, so as long as I'm in the "black" (meaning I keep more plants alive than kill), we're good.
This is such a fun and easy project--creating one's own terrarium. I posted about it here in more detail. All you need is a glass vessel with an opening at the top, some polished river rocks, and 3 small plants. That's it! Try to choose plants that enjoy the same amount of water to increase their longevity.
* I bought the amber polished river rocks at Pottery Barn b/c I love the color so much but I've seen cooler toned rocks at Big Lots and the like as well.