The one thing I've consistently disliked most of all about the publishing process since I first started learning about how it works is the fact that from square one publishing is all about marketing. We have to sell ourselves to agents, who in turn sell us to publishers, who then sell our books to public, and it's probably a good idea to do a little marketing of the book yourself once it hits the shelves. It's all about making sales from the moment your feet hit the ground.
It should be easy enough to understand why this can be aggravating. We write. If we wanted to play the marketing game we'd get a degree in it and go work for some corporation. But getting upset about it won't get you anywhere. Refuse to play the game and you're done. So as long as you're stuck trying to market yourself any way you can, you better take the game seriously.
Regardless of where you are in the process, a good rule of thumb to follow for anything that involves a higher ratio of failure to success is to assume everything will fail.
Just plugged your new book over on twitter? Good. No one's listening. Go make a fan page on Facebook.
As soon as you've finished doing something, that step has failed. Don't wait and see if it has or it hasn't; it has.
The real trick is to keep yourself going. It's easy to say "If I'm going to fail at everything, I might as well not bother in the first place". With this, I can't help you. It's your mind, your willpower. Whatever you can do to keep yourself trucking, make sure you do it.
When all's said and done and your book has gone to print, a good thing to bear in mind is to not be afraid of spamming. Of course, this won't be the most popular approach, but if you can find ways of doing it within reason it usually pays off. For example, a good way to increase your count of subscribers is to attach some of your videos as video response to the most popular videos of the week and of all time. Running into the comments every few hours and telling people to check out your channel can work, but less people will be willing to see what's going on.
This can be harder to pull off with marketing books, but there are still ways it can be done. Find a way of slipping some form of ads for it into your local bookstore. Somewhere people will notice when they're actively looking at the area, or maybe even just skimming it, but the staff probably won't notice as not being theirs. Of course, slipping it through like some sort of drug may not be necessary. Try speaking to the manager or someone first, and if that falls through, go ahead and plant it. Just try not to get caught or they may not be so friendly the next time you're in.
Finally, one thing you should definitely do at any step on the road is to set yourself unrealistic goals. As with assuming everything will fail this can be dangerous if you have the wrong mentality, but go into it the right way and the payoff should pull in your favor. If you say "I want to be published" you can get there, but it's going to be a rough ride. For one thing, if you fall short of achieving your goal, you probably don't have a milestone you can look at and still stay positive about. For another, you will have loads of other people all trying to get their book published as well, and may end up bowing to the pressure of competition. But if you say "I want my book to be #1 on the NY Times bestseller list by the end of the year" the only people who will have the same ambition will be idiots.
Odds are very good you won't achieve that number one spot on the list, but if you assume everything will fail and do everything you possibly can to market yourself, you'll have a benchmark you can stop to look at and be proud of. The same goes for finding an agent. If you say "I want an agent" that's great. If you say "I want to be signed by the President of XYZ agenting group" you very well could come up short, but you'll come up short somewhere where you should still be able to be happy with your success.
Anyway, that's all I've got for you today. Hopefully tomorrow the other thing I've got rattling around in my skull will go up.