Not too strong light, and only water if the soil seems dry. It might be helpful to put a little plastic cap over the leaves to add some humidity.

How are your watering habits? The kind of light it is getting? This type of begonia can go into a semi-dormancy but that does not mean treating it like a bulb - do water it so the rhizome stays plump but do not keep it sopping wet else it will lead to rot. A plastic cover often helps if you see the rhizomes starting to get soft - it will keep humidity high and you can cut back on water.

I have a feeling we may have the same rex begonia from Lowe's. I have watered it sparingly the last couple of times and it has not lost a leaf (yet). I keep it next to a southern window and the temps stay nice and warm.

yup that looks like it! it is sitting close to a east window... and gets direct morning sun. i have it sitting on a plate of water to help with the humidity. i dont wanter until the soil feels dry to touch... it is in a 2.5" pot so it dries out pretty quick.

There is a fine line on watering bare rhizomes - too wet (won't dry out) will cause rot and too dry will kill them too. Some sellers grow in a lot of peat (might be good for a greenhouse) but the unsuspecting buyer can water it thoroughly and find out that it won't dry out after it is too late. Removing the root ball and placing it on newspaper is one way to wick away excess water.

The old adage is to insert your finger an inch deep in the pot to check for moisture (I find that extreme but some pots I do resort to at least feeling the surface). If you can feel moisture at that depth then don't water, if dry to the touch then water thoroughly.

You may find in your case (drying out rapidly) that a baggie or a clear cup over the plant will do wonders.

heres some pics of the size and pot and humidity tray i have it in. it is in african violet soil (mixed by myself).. one part soil, one part vermiculite, one part peat.

and heres to get a pic of how big it is.

beautiful! i rather got scared of begonias after this one mostly hit the dust! but you give me hope. i think what killed the original plant was i did have it in a terrarium at first, and i ran out of room and had to take it out of the terrarium and in like 6 hours, it was dead!

your thoughts about soil brings me to another question... is it detrimental to reuse soil? i do... after a period of dormancy... and thought i better ask. :)

Moving plants out of a terrarium without acclimating them to the "new" environment is often a fatal mistake (been there, done that).

Don't give up hope - sometimes you have to have a lot of setbacks to see progress - I know I've had a lot of failures to get where I am now and still have losses. Learn from mistakes. The other thing is sometimes you get a weak plant and there isn't much to do about it - if you try the same plant again and again then maybe it is time to move on to another "hardier" plant.

No, I often re-use the old soil. I often top it off with fresh soil though and may have to stir up the old with the new. If it is friable then I'd say go for it.