Sometimes the motivation and flow only comes after you start. I find this the case for shipping as well.

If I don’t feel like working or notice myself procrastinating, then just shipping something small usually gets the ball rolling. The key is to keep it small. Something you can ship before your self discipline is depleted.

As you ship more and more you build more motivation and start to get into the flow.

Side note: if you’re not able to get into a good flow, maybe it’s just not a good time. It’s important to take care of ourselves mentally and physically. So try not to be too hard on yourself if you’re not feeling productive. Take a walk. Prepare some food. Blow off some steam. Of course keep it within reason (for me watching TV during work time is a no-no, but a walk or reading is fine).

For me "shipping" as a process is not really motivating, usually I'm struggle a lot. Because what is actually "shipping" is? Writing a code for your side-project? For me motivation is the final result (not the process of itself). So I know why and what I'm doing. Or at least I think that I know :D.
To be able to get in the flow, I start to thinking about this final result (what and why) and this helps, but not all the time. Most of the time I'm watching YouTube :D.

Every time it's hard time for me, I remember this guy

For me its been to build in small increments. Get something out very quickly specially things such as the first release comes with, that is email/domain/ etc. Once thats out of the way I try to get some users in and then just release small increments every couple of days. Still trying to stick to this routine, but small incremental changes each day no matter how small will get the ball rolling.