You need little bit fat to make momo juicy

1. animal fat is not good so avoiding using fat mixed meat. 

2. either use little butter or ghee while preparing the meat

3. Best is to use good quality olive oil during meat preparation, some olive oil are mixed olive oil don't use those,

4. As Magorkhe1 said adding little bit soya sauce also makes  momo juicy

5. Use of  soya sauce and olive oil some ppl dont like becua eof the taste.

6. As Rahulbhi said use of chhyapi gives momo a Kathmandu taste