Keeping contact, yes. Very important.
You need to keep contacting these bosses, to update about their business, to have a tea. It's good for any freelancers.
I just came to know that! Keep good relationship with previous bosses will help too.

Once they are stuck with your design, they won't/aren't like to take risk hiring another freelancer (Not know the skills and promises on delivery)

50% upfront is a lot, 30% is more like it.
But if you have nothing to do/no income at the moment, submitting low-res work is not a problem. You get experience by then.
Payment is not everything, but with payment, you will do it with grin!