Are you getting new leaves and buds together with the leaf yellowing? Sometimes when the Chinese Elm doesn't get cold enough, it will hold onto all its leaves through the winter, and then push them off at once (yellow & fall) in the spring as the new buds form.
If not, take a look at the watering suggestions Jay made. More info in the ***FAQ*** and
bonsaiTALK Links under Beginners and Techniques->Watering
Regards,
Matt