17. Use the more options button. It lets you add additional text boxes which are extremely useful and serve multiple purposes. You can change the font, but in my opinion in most cases for most memes the default Impact font is perfect. You can change the font size. You can toggle off font shadow, ALL CAPS, and if you're on mobile this is where you can enable drag/drop and resize which is always available on desktop. You can also use original resolution of a template image, but in my opinion bigger pictures can lead to users skipping over if you can't read everything at one time.
18. As mentioned, adding text boxes and using additional text boxes can be impactful. Did the lowercase "imgflip" in my meme's main picture catch your eye? To do that I used two different boxes. Manually typed everything in all caps except "img" and used a second text box to make font red for "flip".
19. I can't help you with Custom Templates that you'll use only once, those are always going to be a gamble as to when they'll actually feature. Feature times seem to depend on what imgflip mod is working at the time. However, IF YOU'RE GOING TO USE A CUSTOM TEMPLATE MORE THAN ONCE, there's a great way to guarantee that AFTER YOUR FIRST SUBMISSION, your future submissions with the template will post within 2 hours.
If you save an image to the public directory (so that anyone can use it, and then it shows up in your "My Templates" tab), and then use that image in a submission initially, then from then on out it doesn't get treated like a custom submission. Once it's been approved and featured once, it's what I consider an "established template" and established templates get featured just as quickly as the popular templates.
The same works for when you're searching the img template database. When you click on the template you can figure out if it will post right away, or be treated like a custom submission based on if it has any featured memes tied to it. An example of this (as of 11/13/16) is "sniper dog." Right now there's 3 "sniper dog" templates out there, and two of them don't have any features. These would be treated as custom templates. If you find the 1 that has my "Grumpy Cat Got You Down?" caption, that would get posted right away. LONG STORY SHORT, if you see multiple versions of the image you want to use, use the one that already has been featured!