What do you think is the best way to ask someone to sign model release? How do you explain that you want to earn some money on them? 

What sort of a situation are you in?

If it's family members or friends, tell them you'll buy them a beer in return for them signing.  (I have an easy going social circle!)

If it's a professional model, they will expect to sign a release.

If it's a person you don't know, think long and hard.  I wouldn't sell RF pictures of someone I didn't know.

Beer!  It is a very good idea.   They'll do everything for a good glass of it!

I see that the lifestyle photography is a way to go.. Competition is tremendous but I can always try to make something.