Not sure if it's too late to post here, but I have a few minor tips as an avid procrastinator myself.
One way to get them going is by setting aside time on a regular basis or setting some sort of schedule for when you can work on something. Making something habitual, like doing yoga for 5 minutes a day in the morning, or going to the gym twice a week on set dates, makes those tasks much easier to do than it might be initially getting started.
Depending on what motivates you, you could set yourself a reward system of sorts. When I need to finish, say, a 5 page paper in two days, I can tell myself that if I finish it on day one, I can laze about on day two, or work on other, easier projects.
Speaking of easier projects, depending on how you work, it might be easier to delegate time to tasks you may think are more difficult, or that will take longer, first, so as to get them out of the way in order to focus on easier ones later, as mentioned in the previous point. Doing them the opposite way may be helpful as well, however, as going through several easy projects and building up to a "grand finale" may help merely by getting you started, motivated, and warmed up for what is to come.
You can't really underestimate the power of one's mindset when going into a project. It might sound cheesy, but genuinely thinking that "xyz" will be fun and interesting, or that you are not procrastinating, rather than working your way up the ladder to that "difficult" commission can improve the way you approach and handle it when it comes. So, thinking it will be "difficult" itself is setting you up for failure and a bad experience, whereas telling yourself you're going to work on a new and challenging commission where you'll have the opportunity to expand your art technique and learn new aspects of how to achieve this or that, will have exponentially better results.
To add onto that, your mindset and the questions you ask yourself can literally affect your decisions, for better or worse. Take the phrase, "Oh, this is going to be extremely difficult, I bet other artists also struggle with hands," may lead one to crawling on the internet forums to talk about how the extraordinary difficulty of hands. Instead, telling oneself, "Oh! I know I can improve in this area because I've seen others do it, and this piece gives me a great opportunity to work on it!" may instead lead this person to, motivatedly, doing warmup hand sketches before getting around to drawing Shiva, or The Master Hand, or whatever else they're being commissioned for.
Working your way up to things, like delegating tasks from easier to challenging, may help when you're "not feeling it". Work on other things, or related things, and maybe then working your way back to the commission itself when comfortable might help. This is like practicing warmup music before playing full compositions, or stretching your muscles before doing a sport or rigorous activity, because it primes you for what is ahead, and can even just get the ball rolling in the right direction.
Lastly, lists! Make a list of tasks so you can keep track of what needs to get done, and you can motivate yourself simply by checking the items off as you go, because it creates a sense of finality. Maybe treat yourself as you go along to aid in the process. Breaking up larger tasks into bite-sized chunks helps to get through everything. Trying to eat a cake whole is a vain endeavour with our set jaws, since we don't have danger noodle privileges, so we can help ourselves to using utensils instead, and in the end, we still make plenty of progress. That kid from Matilda can attest.
I hope this was helpful! Though it was very late, ahaha.
Winter 
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